When adding double digit numbers, it is important to start on the right side before moving on to the left.
In this case, it's the numbers 5 and 7. 5 plus 7 equals 12. Only 2 will be written down underneath the right side. Since there is a leftover "1" it must be carried over to the left side of the equation and add it to that side.
2 plus 1 has now become 3 plus 1. Find the sum of the “new” equation, 3 plus 1, and write down the sum of this equation underneath the left side. The answer for 25 plus 17 is 42!