Solve for .
Collapse the constant product first. The left-hand side hides a plain multiplication: . Doing this before touching the keeps the equation readable:
Recognise the shape of the problem. The unknown appears on both sides, so this is a gross-up: some fixed amount plus of the total equals the total. The standard move is to collect all terms on one side.
Subtract 0.13x from both sides. Factoring out is the key step - is the fraction of the total that the fixed amount represents.
Divide by 0.87. The exact value is the fraction , which does not terminate, so round only at the end.
Check the answer by substitution. , and - the two sides agree.
Note the scaling shortcut. Because the equation is linear and homogeneous in the constant, scaling the constant scales by the same factor: the same equation with gives , exactly one millionth of this answer.
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