Question

Calculate the area of a circle with a dime tier of 16cm pi=3

Answer 1

The area of every circle is        (pi) (radius²) .

Radius = 1/2 of the diameter, so the area of your circle is

                   (pi) x (8 cm)²  =  201 cm² .    (rounded)

(pi) is not 3 .   There's nothing wrong with approximating (pi),
but 3 is more than 4.5% wrong, and that's too much.  There's
no reason why  3.14  should be too hard to handle.

Answer 2

A=πR²
d=2R 
d=16 => R=8 
===> A=3*8²=3*64=192 cm²