What number plus eight equals 20?
In this problem, the number that we need to find is at the start of the calculation. What number is it that if we add eight to it the answer will be 20? We can sketch a bar model to help us understand what the question is asking. This bar represents the 20 in our calculation, the result of adding our mystery number and eight. Here is the eight that we add to our mystery number and here is the number that we need to find. Let’s point to each part of the bar model to show that we understand the problem.
What number plus eight equals 20? To find the answer, we could start with 20 then take away eight. Because if we subtract eight from 20, the number that we’re left with will be the number that pairs with eight to make 20, which is the number we’re looking for. What we’ve done is start with 20 and work backwards. Instead of adding eight, we need to do the inverse or the opposite. We need to take away eight. 20 take away eight equals the number we’re looking for.
What number is left if we take away eight from 20? Well, 20 take away eight equals 12. And so, we know 12 add eight equals 20. The number that pairs with eight to make 20 is one ten and two ones or 12.
And so, we know 12 plus eight equals 20.