Other income (expense), net. Other income, net for the three months ended March 31, 2024 was $7.7 million as compared to $4.6 million in the three months ended March 31, 2023. The increase in other income, net was primarily due to the increase in our investments in marketable securities and the returns on those investments year over year.
Income tax expense. Income tax expense was $24.7 million for the three months ended March 31, 2024, or 26.1% of income before income taxes, as compared to an expense of $19.5 million, or 26.0% of income before income taxes for the same period in the prior year.
Comparison of the Nine Months Ended March 31, 2024 and 2023
Revenues. Our revenues for the nine months ended March 31, 2024 were $1,505.9 million, representing an increase of $152.0 million, or 11.2%, from $1,353.9 million for the same period in the prior year. General Education revenues increased $106.1 million, or 12.7%, year over year. The increase in General Education revenues was primarily due to the 8.1% increase in enrollments, school mix (distribution of enrollments by school), and other factors. Career Learning revenues increased $45.9 million, or 8.9%, primarily due to a 10.2% increase in enrollments and school mix.
Instructional costs and services expenses. Instructional costs and services expenses for the nine months ended March 31, 2024 were $930.5 million, representing an increase of $51.6 million, or 5.9%, from $878.9 million for the same period in the prior year. This increase in expense was due to the timing of hiring of personnel and salary increases. Instructional costs and services expenses were 61.8% of revenues during the nine months ended March 31, 2024, a decrease from 64.9% for the nine months ended March 31, 2023.
Selling, general, and administrative expenses. Selling, general, and administrative expenses for the nine months ended March 31, 2024 were $399.5 million, representing an increase of $36.1 million, or 9.9% from $363.4 million for the same period in the prior year. The increase was primarily due to an increase of $13.0 million in bad debt expense, $9.9 million in professional fees and marketing expenses, and $9.8 million in personnel and related benefit costs, including stock-based compensation. Selling, general, and administrative expenses were 26.5% of revenues during the nine months ended March 31, 2024, a decrease from 26.8% for the nine months ended March 31, 2023.
Other income (expense), net. Other income, net for the nine months ended March 31, 2024 was $19.4 million as compared to $9.6 million in the nine months ended March 31, 2023. The increase in other income, net was primarily due to the increase in our investments in marketable securities and the returns on those investments year over year.
Income tax expense. Income tax expense was $48.4 million for the nine months ended March 31, 2024, or 25.5% of income before income taxes, as compared to an expense of $30.9 million, or 27.0% of income before income taxes for the same period in the prior year. The decrease in the effective tax rate for the nine months ended March 31, 2024 was primarily due to the tax impacts of non-deductible compensation and Tallo transaction, which were partially offset by the decrease in excess tax benefit of stock-based compensation.
Liquidity and Capital Resources
As of March 31, 2024, we had net working capital, or current assets minus current liabilities, of $927.1 million. Our working capital includes cash and cash equivalents of $376.6 million and accounts receivable of $577.8 million. Our working capital provides a significant source of liquidity for our normal operating needs. Our accounts receivable balance fluctuates throughout the fiscal year based on the timing of customer billings and collections and tends to be highest in our first fiscal quarter as we begin billing for students. In addition, our cash and accounts receivable were significantly in excess of our accounts payable and short-term accrued liabilities at March 31, 2024.
During the first quarter of fiscal year 2021, we issued $420.0 million aggregate principal amount of 1.125% Convertible Senior Notes due 2027 (“Notes”). The Notes are governed by an indenture (the “Indenture”) between us and U.S. Bank National Association, as trustee. The net proceeds from the offering of the Notes were approximately $408.6 million after deducting the underwriting fees and other expenses paid by the Company. The Notes bear interest at a rate of 1.125% per annum, payable semi-annually in arrears on March 1st and September 1st of each year, beginning on March 1, 2021. The Notes will mature on September 1, 2027. In connection with the Notes, we entered into privately negotiated capped call transactions (the “Capped Call Transactions”) with certain counterparties. The Capped Call Transactions are expected to cover the aggregate number of shares of the Company’s common stock that initially underlie the Notes, and are expected to reduce potential dilution to the Company’s common stock upon any conversion of Notes