Tracy is taking advantage of a 25% off sale to buy a new coat. If the coat's sale price is $135, what was the original cost?

Let X be the original cost of the coat.

After the 25% off, the sale price is 0.75X.
According to the problem, 0.75X = $135.
Dividing both sides by 0.75, we get X = $180.
Therefore, the original cost of the coat was $180.