Remember that when we use variables (letters) in math problems, the variables represent unknown numbers. We are not always able to solve for the variables, but sometimes we can. When a comp­arison problem has two numbers and one variable, you can solve for the variable.

Here's a problem we can solve. It tells about two numbers and a variable: M equals 13 more than 45. The first thing to do is to look for the number that comes just before the word "more" or "less." This number will always be a lesser (smaller) number in the number family. In this problem, that number is 13.

Now let's find the other numbers in the family. In this problem, is M the greatest (largest) number or a lesser number? M is more than 45, so M equals the greatest number, and 45 equals the lesser number. We have our complete number family: 13 and 45 equal the lesser numbers, and M equals the greatest number.

We can figure out what number M equals. M equals the greatest number. What do we do to figure out a missing greatest number? We add the two lesser numbers. In this case, we add 13 and 45 to equal 58. That's what M equals. So we cross out M and write the 58.