#### Answer

$\frac{5}{64}$ in

#### Work Step by Step

Lets consider the diameter of the shaft is $\frac{7}{8}$ in
We have to find the length that must be reduced so that the diameter of the shaft is $\frac{51}{64}$ in.
Therefore length = $\frac{7}{8}$ in - $\frac{51}{64}$ in.
= $\frac{7}{8}\times$ $\frac{8}{8}$ - $\frac{51}{64}$
= $\frac{56}{64}$ - $\frac{51}{64}$
=$\frac{5}{64}$ in