Two sample intervals for proportions

A political science professor would like to estimate the difference in the proportion of young voters and older voters in his state who believe that Congress has the same priorities as them. **If the professor wants his $90\%$ confidence interval to have a margin of error of no more than $0.08$, about how large should his sample be, assuming that he selects the same number of younger and older voters?** [[☃ radio 1]]