Write the missing number in the box. 13 plus six is equal to 10 plus what?
We have to find the missing number. Our number sentence has two calculations. And we know that both calculations are equal. We can find the missing number by working out what 13 plus six is. What number is six more than 13?
Let’s start at 13 and count forward six: 14, 15, 16, 17, 18, 19. Six more than 13 or 13 plus six is 19. We know that both calculations are equal. So if 13 plus six is 19, we know that the total of the second calculation is also 19. What do we add to 10 to make 19? The answer is nine. 13 plus six is 19. 10 plus nine is 19. 13 plus six is equal to 10 plus nine. The missing number is nine.