The ratio of the radii of two circles is 9: 16. then what is the ratio of their circumferences?

56.52:100.48

circumference is the diameter × π
diameter is radius × 2

first circle ->

r = 9
d = 9 × 2 = 18
c = 18 × π ≈ 56.52

second circle ->

r = 16
d = 16 × 2 = 32
c =  32 × π ≈ 100.48

so the ratio of circurfremce for 2 circles that have a ratio of radii 9:16 would be 56.52:100.48