Let x = the amount of grapes that can be bought for 0.40.
Note: By 90 I assume you mean 90 cents. The same for 40.
12/x = 0.90/0.40
0.90x = 12(0.40)
0.90x = 4.80
x = 4.80 ÷ 0.90
x = 5.33333
We can round the decimal to the ones place to get 5 grapes.
To solve for S, you would want to get rid of the denominator so multiply 360 onto both sides. then the equation is 360A=pi times r squared times S. Divide both sides by pi and r squared and you get S=360A/pi times r squared