To solve for \( x \) in the equation \( 20 = 5 - \frac{3}{8} x \), follow these steps:
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Start with the equation: \[ 20 = 5 - \frac{3}{8} x \]
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Subtract 5 from both sides to isolate the term with \( x \): \[ 20 - 5 = -\frac{3}{8} x \] \[ 15 = -\frac{3}{8} x \]
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Multiply both sides by -1 to make the \( x \) term positive: \[ -15 = \frac{3}{8} x \]
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To isolate \( x \), multiply both sides by the reciprocal of \(\frac{3}{8}\), which is \(\frac{8}{3}\): \[ x = -15 \times \frac{8}{3} \] \[ x = -\frac{120}{3} \] \[ x = -40 \]
Thus, the solution is: \[ \boxed{-40} \]