# RE2 | anthropic/claude-opus-5 | run 2

## Prompt

After a 20% discount, a jacket costs $40. What was the original price? Show your work.

## Response

The original price was **$50**.

## Work

Let \( x \) be the original price. A 20% discount means the customer pays 80% of \( x \):

\[ 0.80x = 40 \]

\[ x = \frac{40}{0.80} = 50 \]

## Check

- 20% of $50 = \( 0.20 \times 50 = 10 \)
- $50 − $10 = $40 ✓

A common mistake is adding 20% back to $40 (giving $48), which is wrong because the discount was taken off the *larger* original price, not the sale price.

Want a quick formula for reversing any percentage discount?
