Video Transcript
What number is missing? Three plus 99 plus what equals 110.
Three numbers add together to make 110. We know that one of them is three. One of them is 99. And we need to find the third number. There are lots of ways we could do this. For example, we could start with 110, then take away 99, then take away three. And whatever is left will be our missing number. But instead, let’s use a method that involves using a bit of reasoning. The number 99 is very close to a number that would be very helpful to us if we were to work out this missing number.
If we could turn the number 99 into 100, then we’ll find it a lot easier to work with. What if we partition the number three into two and one because, of course, two plus one equals three. So we can now write the addition of two plus one plus 99 plus what. Why did we need to partition the number three like this? Well, it means that we can combine the one with the 99. And this gives us the number 100.
Now, we can find it much easier to find the missing number. Sometimes when we’re adding numbers together, if we juggle them around, it can make the addition a little bit easier. So if we start with 100, we can see that 100 plus two plus something equals 110. So we know that these two numbers must equal 10. 100 plus 10 equals 110. So in the final step, we need to ask ourselves, what do we add to two to make 10? Two plus eight equals 10.
Let’s check our answer by using subtraction, what we said at the start. We’ll begin with 110. And first we’ll take away 99 by taking away 100 and then adding one because we’ve taken away one too many. That gives us the answer 11. So to find the missing number, we just need to subtract three. 11 take away three equals eight. Three plus 99 plus eight equals 110. The number that is missing is eight.
