The first 3 common multiples of 5 and 15 are 15, 30, and 45. Remember that the number itself is also a multiple of any number, since you can multiply it by 1.
The straight lines equal 180 and you subtract 180 to 150 and 180 to 100 to get 30 and 80 and the inside of a triangle is 180 and you subtract 30 and 80 to 180 to get 70.