Rachel had to complete 31 pages of math homework. If she had to complete 12 more pages of math homework than reading homework How many pages did she have to complete in all?
1) Rachel had to complete 31 pages of math homework. 
2) If she had to complete 12 more pages of math homework than reading homework, that means she had to complete 12 more pages of math homework than she did reading homework. 
3) So if we add 12 to the number of pages of reading homework she had to complete, we will get the total number of homework pages she had to complete. 
4) We don't know how many pages of reading homework she had to complete, so we'll just call that number "x." 
5) So we can say that the equation looks like this: 

x + 12 = 31

6) We can solve for x by subtracting 12 from both sides of the equation. 

x + 12 - 12 = 31 - 12

7) This leaves us with just x on the left side of the equation, and 19 on the right side of the equation. 

x = 19

8) So Rachel had to complete 19 pages of reading homework. 
9) If we add that to the 12 pages of math homework she had to complete in addition to the 31 pages she was already completing, we get a total of 31