TeX source:

\begin{aligned}\text { Total Change in the M1 Money Supply } & =\frac{1}{\text { Reserve Requirement }} \times \text { Excess Requirement } \\& =\frac{1}{0.10} \times \$ 9 \text { million } \\& =10 \times \$ 9 \text { million } \\& =\$ 90 \text { million }\end{aligned}