1. **State the problem:** Three girls paid 100 each, totaling 300, for a motel room. The correct charge was 250, so the clerk gave 50 to the attendant to return to the girls. The attendant gave 10 back to each girl (total 30) and kept 20. The girls effectively paid 90 each, totaling 270, plus the attendant's 20, which seems to sum to 290. The question is: where is the missing 10? 2. **Analyze the payments:** The girls originally paid 300. 3. **Refund given:** The attendant returned 10 to each girl, so each girl paid 100 - 10 = 90. 4. **Total paid by girls now:** 3 × 90 = 270. 5. **Breakdown of the 270:** This 270 includes the 250 that the motel keeps plus the 20 the attendant kept. 6. **Common mistake:** Adding the attendant's 20 to the 270 is incorrect because the 270 already includes that 20. 7. **Correct accounting:** - Motel's share: 250 - Attendant's share: 20 - Girls' refund: 30 (3 × 10) Sum: 250 + 20 + 30 = 300, which matches the original amount. 8. **Conclusion:** There is no missing 10. The error is in adding the attendant's 20 to the 270 instead of subtracting it from 300 to find the correct amount paid by the girls. **Final answer:** The missing 10 is a result of incorrect addition; the 270 already includes the attendant's 20, so no money is missing.