It's the sum of 42 and 13 which is 55.
20
19
the sum of 67 + 13 = 80
13.
6 + 7 = 13(Sum means add)
10
The sum of the first 13 odd numbers is 169.
-13
26
n + 13
Let 'N' be the number. The sum of the number and 13 is (N+13).The square of the sum of the number and 13 is (N+13)2N2 + 26 N + 169.It's probably easier to add 13 to the number and then square that answer ... less arithmetic.