Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

Let $F$ be the force provided by one rubber band. With the information in the question, we can set up a force equation: $2F = ma$ $F = \frac{ma}{2}$ Let $n$ be the required number of rubber bands. Therefore; $nF = (\frac{m}{2})(3a)$ $(n)(\frac{ma}{2}) = \frac{3ma}{2}$ $n = 3$ Three rubber bands are required.