To solve for \( x \) in the equation
\[ 20 = 5 - \frac{3}{8}x, \]
let's first isolate the term with \( x \). Start by subtracting 5 from both sides:
\[ 20 - 5 = - \frac{3}{8}x. \]
This simplifies to:
\[ 15 = -\frac{3}{8}x. \]
Next, to solve for \( x \), multiply both sides by -1:
\[ -15 = \frac{3}{8}x. \]
Now, to eliminate the fraction, multiply both sides by the reciprocal of \(\frac{3}{8}\), which is \(\frac{8}{3}\):
\[ -15 \cdot \frac{8}{3} = x. \]
Calculating this gives:
\[ x = -\frac{120}{3} = -40. \]
Thus, the solution for \( x \) is
\[ \boxed{-40}. \]
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