same period in the prior year. This increase in expense was due to hiring of personnel in growth states and salary increases. Instructional costs and services expenses were 60.8% of revenues during the three months ended September 30, 2024, a decrease from 64.0% for the three months ended September 30, 2023.
Selling, general, and administrative expenses. Selling, general, and administrative expenses for the three months ended September 30, 2024 were $168.5 million, representing a decrease of $1.1 million, or 0.6% from $169.6 million for the same period in the prior year. Selling, general, and administrative expenses were 30.6% of revenues during the three months ended September 30, 2024, a decrease from 35.3% for the three months ended September 30, 2023.
Interest income (expense), net. Net interest expense for the three months ended September 30, 2024 was $2.4 million as compared to $2.1 million for the same period in the prior year. The increase in net interest expense was primarily due to our finance leases.
Other income (expense), net. Other income, net for the three months ended September 30, 2024 was $8.8 million as compared to $5.2 million for the same period in the prior year. 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 $11.3 million for the three months ended September 30, 2024, or 21.6% of income before income taxes, as compared to an expense of $1.5 million, or 23.9% of income before income taxes for the same period in the prior year. The decrease in the effective tax rate for the three months ended September 30, 2024 was primarily due to the excess tax benefit of stock-based compensation, which was partially offset by the tax impact of non-deductible compensation.
Liquidity and Capital Resources
As of September 30, 2024, we had net working capital, or current assets minus current liabilities, of $1,060.7 million. Our working capital includes cash and cash equivalents of $317.8 million and accounts receivable of $675.7 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 September 30, 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 and/or offset any cash payments the Company is required to make in excess of the principal amount of converted Notes. The upper strike price of the Capped Call Transactions is $86.174 per share. The cost of the Capped Call Transactions was $60.4 million and was recorded within additional paid-in capital.
Before June 1, 2027, noteholders will have the right to convert their Notes only upon the occurrence of certain events. After June 1, 2027, noteholders may convert their Notes at any time at their election until two days prior to the maturity date. We will settle conversions by paying cash up to the outstanding principal amount, and at our election, will settle the conversion spread by paying or delivering cash or shares of our common stock, or a combination of cash and shares of our common stock. The initial conversion rate is 18.9109 shares of common stock per $1,000 principal amount of Notes, which represents an initial conversion price of approximately $52.88 per share of common stock. The Notes will be redeemable at our option at any time after September 6, 2024 at a cash redemption price equal to the principal amount of the Notes, plus accrued and unpaid interest, subject to certain stock price hurdles as discussed in the Indenture.
On January 27, 2020, we entered into a $100.0 million senior secured revolving credit facility (“Credit Facility”) to be used for general corporate operating purposes with PNC Capital Markets LLC. The Credit Facility has a five-year term and incorporates customary financial and other covenants, including but not limited to a maximum leverage ratio and a