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justkidding1
To use mental math think instead of 81 to 80 and 16 to 10 and subtract that and add the other numbers and subtract that and you will have your answer presto!
1 matches to 6 - subtract any way is 5


81-16=80-15

subtract both by 1


if there's a smaller digit, 2 numbers always add up by x-1

8-1=7
1+x=7
x=6

65

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