[tex]\frac{f}{5}=\frac{f+3}{20}[/tex] Cross multiply. (multiply 5 on both sides, and multiply 20 on both sides)
(20)f = 5(f + 3) Multiply 5 into (f + 3)
20f = 5f + 15 Subtract 5f on both sides
15f = 15 Divide 15 on both sides to get "f" by itself
f = 1
