Selling, General and Administrative. Selling, general and administrative expense, which is primarily comprised of compensation expense, decreased by $1.6 million, or 1.3%, to $121.9 million in the three months ended December 31, 2020, from $123.5 million in the three months ended December 31, 2019. Selling, general and administrative expense decreased as a percentage of revenue from 11.8% in the three months ended December 31, 2019, to 11.2% in the three months ended December 31, 2020. The slight decrease is mainly due to lower selling and marketing and travel costs in the three months ended December 31, 2020 compared to the three months ended December 31, 2019 as a result of the COVID-19 restrictions and was also attributable to decrease in the account receivable allowances. The decrease was partially offset by selling, general and administrative expense, attributable to acquisitions completed in fiscal year 2020. Selling, general and administrative expense may fluctuate from time to time, depending upon such factors as changes in our workforce and sales efforts and the results of any operational efficiency programs that we may undertake.
Amortization of Purchased Intangible Assets and Other. Amortization of purchased intangible assets and other in the three months ended December 31, 2020, decreased by $1.4 million, or 6.7% to $19.9 million from $21.3 million in the three months ended December 31, 2019. The decrease in amortization of purchased intangible assets and other was primarily attributable to a completion of amortization of previously purchased intangible assets, partially offset by an increase in amortization of intangible assets due to acquisitions completed in fiscal year 2020.
Operating Income. Operating income decreased by $3.4 million, or 2.4%, in the three months ended December 31, 2020, to $140.2 million, or 12.9% of revenue, from $143.6 million, or 13.8% of revenue, in the three months ended December 31, 2019. The decrease in operating income was attributable primarily to the increase in research and development expense during the three months ended December 31, 2020. Positive foreign exchange impacts on our revenue, resulting in a positive impact on our operating income.
Interest and Other Expense, Net. Interest and other expense, net, changed from a net expense of $0.4 million in the three months ended December 31, 2019 to a net expense of $6.5 million in the three months ended December 31, 2020. The increase in interest and other expense, net, was primarily attributable to an increase in interest expenses related to financing activities recorded in the three months ended December 31, 2020 compared to the three months ended December 31, 2019.
Gain from sale of a business. Gain from sale of a business, for the three months ended December 31, 2020, was $226.4 million, while there was no such gain in the three months ended December 31, 2019. Please see Note 4 to our consolidated financial statements.
Income Taxes. Income taxes for the three months ended December 31, 2020 were $60.5 million on pre-tax income of $360.1 million, resulting in an effective tax rate of 16.8%, compared to 19.1% in the three months ended December 31, 2019. Absent the gain from sale of a business, the effective tax rate for the three months ended December 31, 2020, would have been 15.6%. Our effective tax rate may fluctuate between periods as a result of discrete items that may affect a particular period. Please see Note 10 to our consolidated financial statements.
Net Income. Net income increased by $183.7 million, or 158.5%, to $299.6 million in the three months ended December 31, 2020, from $115.9 million in the three months ended December 31, 2019. The increase in net income was primarily attributable to the gain from sale of a business, net of tax.
Diluted Earnings Per Share. Diluted earnings per share increased by $1.43, or 168.2%, to $2.28 in the three months ended December 31, 2020, from $0.85 in the three months ended December 31, 2019. The increase in diluted earnings per share was primarily attributable to the gain from sale of a business, net of tax, which increased the diluted earnings per share for the three months ended December 31, 2020, by $1.42, as well as to the decrease in the diluted weighted average number of shares outstanding which resulted from share repurchases. Please see also Note 11 to our consolidated financial statements.
Liquidity and Capital Resources
Cash, Cash Equivalents and Short-Term Interest-Bearing Investments. Cash, cash equivalents and short-term interest-bearing investments, totaled $1,512.9 million as of December 31, 2020, compared to $983.9 million as of September 30, 2019. The increase was mainly attributable to $416.5 million positive cash flow from operating activities, reflecting healthy cash collections and the cash benefit of a multi-year strategic partnership agreement with T-Mobile, $290.8 million net cash received from sale of a business and, $12.7 million of proceeds from stock option exercises, partially offset by $90.0 million repurchase of our ordinary shares, $50.1 million for capital expenditures, net, $43.1 million of cash dividend payment and $9.9 million payments for business and intangible assets acquisitions. Net cash provided by operating activities amounted to $416.5 million and $163.9 million in the three months ended December 31, 2020 and 2019, respectively.
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