Idon't know how to start or do number 21. i'm completely lost. i know the answer is 15 from the answer key.

If we are answering this we will need to use ratios

80÷12=100÷x
x=15

Hope this helps!

The sum of the interior angles of a circle must add up to 360 degrees.
thats how you determined the angles of SR was 100 degrees

360 degrees is equivalent to the total circumference

90 degrees is equivalent to 1/4 of the total circumference (90/360 = 1/4)

hence, if 80/360  is equal to 12, then you could use proportions in order to determine what the corresponding length of SR is

80/360 =12 --->  80/ (360 *12) =1

100/360 = x ---> 100/(360 *x )=1

so 80/(360*12) = 100/(360x)

doing some rearranging and canceling
x/100 = 12/80

solve for x