How do i find out an equation of unit fractions

Given the two similar triangles, how do i find the missing length? if a=4, b=5, and b'=7.5 find a'

An engineer scale model shows a building that is 3 inches tall. if the scale is 1 inch = 600 feet, how tall is the actual building?

In a recent year, the weather was partly cloudy 2/5 of the days. assuming there are 356 days in a year, how many days were partly cloudy?