Choose the One Alternative That Best Completes the Statement or Answers

Multiple Choice

Choose the one alternative that best completes the statement or answers the question.
Solve the problem. Round your answer, as needed.
-A formula for the focal length of a lens is
$\mathrm{f}=\frac{\mathrm{ab}}{\mathrm{b}+\mathrm{a}}$
Calculate $\mathrm{f}$ (the focal length) for $\mathrm{a}=8 \mathrm{~cm}$ and $\mathrm{b}=15 \mathrm{~cm}$ .

A) $5.2 \mathrm{~cm}$
B) $2.8 \mathrm{~cm}$
C) $7.5 \mathrm{~cm}$
D) $4.0 \mathrm{~cm}$