Uncertainty in the stock market has led Sam to diversify his investments. He invested

$$40,000$ in stocks that earned $25\%$ and $$20,000$ in bonds that lost $3\%.$ He invested

some money in mutual funds that earned $13\mathrm{.}4\%.$ If he earned an average rate of

return of $15\%$ on all his investments, how much money did he invest in mutual funds?