40 dimes is $4.00. you would need $1 in nickles. 2 nickles is 10 cents. so you would need 20 nickles.
10x+160=300
-160 -160
10x=140
divide both sides by 10
x=14
$14
The equation of a circle:

(h, k) - a coordinates of the center
r - a radius
A diameter d = 2r
We have

Answer: d = 2 · 10 = 20
3 sig figs in the final answer since the denominator has 3 sig figs. So the answer would be 4.05