To solve for a, divide both sides of the equation by b:
\(ab = \frac{\mathrm{b} }{\mathrm{4} }\)
\(\frac{\mathrm{ab} }{\mathrm{b}} = \frac{\mathrm{b/4} }{\mathrm{b} }\)
\(a = \frac{\mathrm{b} }{\mathrm{4} } \times \frac{\mathrm{1} }{\mathrm{b}} = \frac{\mathrm{1} }{\mathrm{4} } \)