15)
20/8 = x / 6
x = 20 * 6 / 8
x = 15
16)
18/54 = (x+4)/75
1/3 = (x+4) / 75
3(x+4) = 75
3x + 12 = 75
3x = 63
x = 21
SF = 54/18
SF = 3
17)
complement angles sum = 90
90 /18 = 5
so
5 x 11 = 55
5 x 7 = 35
smaller angle = 35 degrees
18)
ratio 4:5:9
4+5+9 = 18
sum of interior angles of a triangle = 180
180 / 18 = 10
largest angle = 10 x 9 = 90
answer
largest angle = 90 degrees