Urgent help needed :(

Hey guys,

its my first time in this forum and I am quite desperate. This task is totally confusing me:

It is believed that as many as 25% of adults over 50 never graduated from high school.
We wish to see if this percentage is the same among the 25 to 30 age group. How many if these younger age groups must we survey to estimate the proportion of non-grads with 90% confidence with a width of the interval of not more than 10%?

Till now I only know how to tackle the problem from the other side, like having the amount of people given and the confidence interval. Could anybody provide me with a hint?

Thanks for helping.