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# Relating addition and subtraction

Video transcript

I have two green balls
and three purple balls so how many balls
do I have altogether? So two green balls
and three purple balls is going to be 5 balls altogether. We can count them;
one, two, three, four, five. Now what if we did something
a little different? Let's start with the five balls,
and let's take away two. Let's subtract two of these balls. Take away one and two,
and what are we left with? We are left with
one, two, three balls. We're left with three balls. You might see a pattern here;
two plus three is equal to five but if you start with five
and take away two, you're going to be left with three. Look how they're arranged. Two plus three is equal to five
and five minus two is three. It works the other way around. If I start with five
and I were to take away three, if I subtract one, two, three,
what am I left with? I'm left with one, two,
I'm left with two. So two plus three is five,
three plus two is five, which is another way of saying
that five minus two is three, or five minus three is two. Take a look at this, it's a really important
idea to understand in math. If you add two numbers
and you get another number, then you take the result
and subtract one of these you're going to get the other number.