To solve the equation \( 20 = 5 - \frac{3}{8}x \), we will first isolate the term containing \( x \).
- Start by subtracting 5 from both sides:
\[ 20 - 5 = -\frac{3}{8}x \]
\[ 15 = -\frac{3}{8}x \]
- Next, multiply both sides by \(-1\) to simplify the equation:
\[ -15 = \frac{3}{8}x \]
- Now, to solve for \( x \), multiply both sides by the reciprocal of \(\frac{3}{8}\), which is \(\frac{8}{3}\):
\[ x = -15 \times \frac{8}{3} \]
- Calculate the product:
\[ x = \frac{-120}{3} = -40 \]
Thus, the solution is:
\[ \boxed{-40} \]