Keiko sold less than three fourths of his sister's sales

The correct question is
Keiko sold 3 less than three-fourths of his sister’s sales. Which expression represents what Keiko sold?

Let
S> the amount that Keiko's Sister sold
K> the amount that Keiko sold

we know that
Keiko sold 3 less than three-fourths of his sister’s sales
K=(3/4)*S-3

the answer is
K=(3/4)*S-3

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