6. $\frac{25}{f + 2}=\frac{5}{3}$

Explanation:

Step1: Cross - multiply

$25\times3 = 5\times(f + 2)$

Step2: Simplify left - hand side

$75=5\times(f + 2)$

Step3: Divide both sides by 5

$\frac{75}{5}=f + 2$ $15=f + 2$

Step4: Solve for f

$f=15 - 2$ $f = 13$

Answer:

$f = 13$