What is the Radius? please help

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

Two cars which are 40 miles apart start moving simultaneously in the same direction with constant speeds. if the speed of the car which is behind is 56 mph and the speed of the other car is 48 mph, how many hours will it take for the car which is behind to catch up the car ahead of it?

Which of the following are independent events? a) knowing that it is going to rain tomorrow, and bringing an umbrella to schoolb) knowing that you have to get up early tomorrow, and going to bed before 9 p. mc) knowing that it is going to rain tomorrow, and going to bed before 9 p. md) knowing that you have a test in school tomorrow, and studying thw night before