segment, the decrease was primarily due to lower net sales from Core Solutions, which declined by $34 million from $277 million in 2017 to $243 million. The decrease was primarily due to decreases in sales relating to disciplines reaching the end of their product lifecycle that are scheduled to be replaced next year with newer programs. Partially offsetting the decrease in our Core Solutions sales was an increase in sales from our Extensions businesses, which primarily consist of our Heinemann brand, intervention, and supplemental products as well as professional services. Extensions businesses net sales for the current period increased $19 million to $207 million in 2018 primarily driven by higher Heinemann net sales. The primary driver of the increase in our Heinemann net sales was sales of theFountas & Pinnell Classroom product, which benefited from additional product launches during the quarter.
Billings: HMH reported billings of $571 million for the third quarter of 2018, up $8 million from the same quarter of 2017. Education and Trade Publishing segment billings for the third quarter of 2018 were $504 million and $67 million, respectively, compared with $512 million and $50 million, respectively, for the same periods in 2017. Our Trade Publishing segment billings increase of $17 million was due to licensing income driven by a new agreement pertaining to our classic backlist titles1984andAnimal Farm. Within our Education segment, the $8 million decline in billings was attributed to a decrease in billings from Core Solutions, which declined by $25 million from $299 million in 2017 to $274 million. The primary driver of the decrease in Core Solutions billings were decreases in billings relating to disciplines reaching the end of their product lifecycle that are scheduled to be replaced next year with newer programs. Billings within our science discipline, which is a new program, increased year over year offsetting some of the older program declines along with a large order from Puerto Rico. Partially offsetting the Core Solutions billings decline was an increase in billings from our Extensions businesses of $16 million to $230 million in 2018, primarily driven by higher Heinemann billings. The primary driver of the increase in our Heinemann billings was billings of theFountas & Pinnell Classroom product, which benefited from additional product launches during the quarter.
Cost of Sales: Overall cost of sales decreased 3%, or $7 million, to $238 million in the third quarter of 2018 from $245 million in the same period of 2017, primarily due to a $7 million reduction in publishing rights andpre-publication amortization.
Selling and Administrative Costs: Selling and administrative costs increased $3 million to $176 million for the third quarter of 2018 from the same period of 2017, primarily due to variable expenses such as commissions attributable to the Heinemann billings, and depository fees which is driven by geographic sales mix.
Operating Income: Operating income for the third quarter of 2018 was $92 million, a $3 million improvement from the same period of 2017 primarily due to aforementioned changes in cost of sales.
Net Income: Net income of $86 million in the third quarter of 2018 was $4 million, or 5%, lower compared to a net income of $91 million in the same quarter of 2017. Net income from continuing operations in the third quarter was $84 million, a $5 million decrease from the $89 million in the same quarter of 2017, due primarily to the same factors impacting operating income and an unfavorable change in our tax provision of $6 million, from a benefit of $10 million for the same period in 2017 to a benefit of $4 million in 2018. The reduction in benefit was primarily due to the Tax Cuts and Jobs Act of 2017 (“2017 Tax Act”) resulting in a lower annual effective tax rate and our ability to utilize indefinite-lived deferred tax liabilities as a source of future taxable income in our assessment of realization of deferred tax assets. Income from discontinued operations, net of tax, increased by $1 million to $2 million from the same period in 2017.
Adjusted EBITDA from continuing operations: Adjusted EBITDA from continuing operations for the third quarter of 2018 was $160 million, a $1 million unfavorable change from $161 million in the same quarter of 2017, primarily due to the aforementioned changes in selling and administrative costs.
Nine Months Ended September 30, 2018 Financial Results:
Net Sales: HMH reported net sales of $1,073 million for the first nine months of 2018, down $20 million from the same period of 2017. The net sales decrease was driven by a $29 million decrease in our Education segment, offset by a $9 million increase in our Trade Publishing segment. Within our Education segment, the decrease was primarily due to lower net sales from Core Solutions, which declined by $63 million from $519 million in 2017 to $456 million. The primary drivers of the decrease in Core Solutions sales were decreases in sales to disciplines reaching the end of their product lifecycle that are scheduled to be replaced next year with newer programs. Net sales within our science discipline, which is a new program, increased year over year offsetting some of the older program declines. Also contributing to the decline in Core Solutions was thenon-recurrence of the $5.0 millionone-time fee we recognized in 2017 in connection with the expiration of a distribution agreement. Partially offsetting the decrease in our Core Solutions sales was an increase in sales from our Extensions businesses of $34 million from $444 million in 2017 to $478 million in 2018 primarily driven by higher Heinemann net sales largely attributed to theFountas & Pinnell Classroom product, which was introduced in the third quarter of 2017, and additional product launches during the third quarter of 2018. Within our Trade Publishing segment, the increase was primarily due to licensing income driven by a new agreement pertaining to our classic backlist titles1984andAnimal Farm. The increase was partially offset by a decrease in ebook sales.
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