To solve the equation \( f + 9.9 = 15.9 \), you need to isolate \( f \).
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Subtract \( 9.9 \) from both sides of the equation: \[ f + 9.9 - 9.9 = 15.9 - 9.9 \]
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This simplifies to: \[ f = 15.9 - 9.9 \]
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Now, perform the subtraction: \[ f = 6.0 \]
Thus, the solution is: \[ f = 6.0 \]