Because each amount has an equal chance of being picked, multiply each $ amount by 1/5 and add them all together. Then since he pays 30% in taxes multiply the previous sum by (1-.3) to find the average after taxes.
//Comment AJ Hartnett
I'm not too sure but I think that you should multiply by .7 before you multiply each by 1/5th and add them all together. In this specific case it obviously works out to be equal, but had you been doing something else more complex, I'm not sure they would...like when we did that problem regarding the average rate to finish a mile and the average time to finish a mile in class.
