To simplify the expression \( 5(a - 3b) - 4a + b \), we will follow these steps:
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Distribute \( 5 \) into \( (a - 3b) \): \[ 5(a - 3b) = 5a - 15b \]
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Substitute this back into the expression: \[ 5a - 15b - 4a + b \]
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Combine like terms:
- Combine the \( a \) terms: \[ 5a - 4a = a \]
- Combine the \( b \) terms: \[ -15b + b = -14b \]
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Now, putting it all together, we have: \[ a - 14b \]
Thus, the expression \( 5(a - 3b) - 4a + b \) simplifies to \( \boxed{a - 14b} \).