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Answer: 21
18 + 15 + 11 + 14 = 58
10 + 11 + 17 = 38, 20 missing to make 58
18 + 13 + 20 = 51, 7 missing to make 58
7 + 16 + 14 = 37, 21 missing to make 58
An investigation involving adding and subtracting sets of consecutive numbers. Lots to find out, lots to explore.
Investigate how you can work out what day of the week your birthday will be on next year, and the year after...