The pictures of this problem are given below. In every picture the unit is represented as a square. So, we have the first equation:
7 + 7
The number seven is given in each term of this equation. So, in the first picture we have seven red squares and seven blue squares. The total number of squares is fourteen, so:
7 + 7 = 14
The second equation establishes that:
7 + 8
This equation is represented in the second figure, but we can say that this equation is the same as:
7 + 7 + 1
or:
(7 + 7) + 1
This new equation is represented in the third figure. In this way, the sum (7 + 7) is known for the previous equation, this sum is 14. Therefore:
(7 + 7) + 1 = 14 + 1 = 15
