#### Answer

$15$ balls

#### Work Step by Step

The "balls per layer" formula gives
$a_{5}=\displaystyle \frac{5(5+1)}{2}=15$

Published by
McDougal Littell

ISBN 10:
0618595414

ISBN 13:
978-0-61859-541-9

$15$ balls

The "balls per layer" formula gives
$a_{5}=\displaystyle \frac{5(5+1)}{2}=15$

You can help us out by revising, improving and updating this answer.

Update this answerAfter you claim an answer you’ll have **24 hours** to send in a draft. An editor
will review the submission and either publish your submission or provide feedback.