On a school trip, one teacher is needed for every group of 10 or fewer students. 72 students want to go on the trip. Lexi tries to work out how many teachers are needed.
Her working is:
72 ÷ 10 = 7.2
7 teachers are needed.
What is wrong with her answer?

Lexi's calculation 72 ÷ 10 = 7.2 is mathematically correct, but her interpretation of the result in the context of the problem is wrong. The result 7.2 means that 7 full groups of 10 students can be formed, with 0.2 of a group left over. This remainder represents 2 students (0.2 × 10 = 2). The rule states that "one teacher is needed for every group of 10 or fewer students". The 7 full groups require 7 teachers. The remaining group of 2 students, although smaller than 10, still requires one teacher. Therefore, the total number of teachers needed is 7 + 1 = 8. Lexi made the mistake of rounding down (or truncating) 7.2 to 7, which leaves 2 students without a supervisor. In this type of real-world problem, you must always round up to ensure everyone is accounted for.