Below is the formula for the circumference of a circle
C = 2πr
This question gives us the diameter. To find the radius (r) you would divide the diameter by two like so...
16.4/ 2 = 8.2
Plug what you know into the formula and solve...
π = 3.14
r = 8.2
C = 2(3.14)(8.2)
C = 6.28(8.2)
C = 51.496
In the question it asks to round to the nearest tenth like so...
51.5
Hope this helped!
~Just a girl in love with Shawn Mendes