3
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Professor Devonshire was losing his marbles, Dr. Wallaby thought, staring at the equations filling the board in Dr. Devonshire’s office. What kind of addition was this?

1+1 = 6

2+1 = 6

1+6 = 6

2+6 = 6

6+6 = 6

1+3 = 8

7+9 = ?

I wonder what goes in the blank?

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5
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what goes in the blank is

9

Because

the addition is of the number of letters in each one of the numbers to the left of the equal sign.

So

one + one = 6 letters
two + one = 6 letters
one + six = 6 letters
two + six = 6 letters
six + six = 6 letters
one + three = 8 letters
and
seven + nine = 9 letters

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