Reverse subtraction
What is the value of c in this subtraction?
Age
11 to 14
Challenge level
Problem
In this subtraction, a, b and c are digits, and a is less than b.
What is the value of c?
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If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.
Student Solutions
Answer: c = 3
a is less than b so:
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11 - 5 = 6, a = 1, b = 5
12 - 6 = 6, a = 2, b = 6
13 - 7 = 6, a = 3, b = 7
14 - 8 = 6, a = 4, b = 8
15 - 9 = 6, a = 5, b = 9
a = 1, b = 5, 51 - 15 = 36
a = 2, b = 6, 62 - 26 = 36
a = 3, b = 7, 73 - 37 = 36
Always get 36 (because both numbers in the sum increase by 11 each time)
So c = 3