Solve for f
f=0
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f\times 9=5f
Multiply 5 and 1 to get 5.
f\times 9-5f=0
Subtract 5f from both sides.
4f=0
Combine f\times 9 and -5f to get 4f.
f=0
Product of two numbers is equal to 0 if at least one of them is 0. Since 4 is not equal to 0, f must be equal to 0.
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