Video: Adding Two Numbers up to 99 by Counting On

50 people are going to a party. Then, 15 more people decide to go. How many people are going to the party? Find the number by using the chart to count.

02:39

Video Transcript

50 people are going to a party. Then, 15 more people decide to go. How many people are going to the party? Find the number by using the chart to count.

Let’s start by underlining the important information in this problem. We start with 50 people, who are all going to a party. And we can see that the number 50 is circled in the chart, ready for us to use too, which is helpful. Then, we’re told that 15 more people decided to go to the party too. So to find out how many people, we need to add 15 to 50.

This is a problem that we can solve by using addition. But the question tells us the exact method that we need to use. It tells us to find a number by using the chart to count. Because we’re adding, we need to count forwards. One way we could find the answer is to start on 50 and to count on 15 squares, remembering that when we get to the end of the row, we start the new row on the left: one, two, three, four, and so on.

But this seems to be taking a little bit of our time. Is there a quicker way? Yes, there is a quicker way. We can see that this chart looks a little bit like part of 100 square. It has 10 numbers in each row. And this means that we can count in tens quickly, just by going up or down in the chart.

So if we want to add 10 to 50, we move downwards: 50 add 10 equals 60. And if we want to subtract 10, we move upwards: 50 take away 10 equals 40. So jumping up or down is a very quick way to subtract or to add 10. We need to add 15. So we can partition 15 into one 10 and one five.

First, we can add 10 really quickly by jumping down one square for one 10: 50 plus 10 equals 60. And now, we just need to add five. We don’t even need to count each individual square because we’re on 60. 60 plus five we know is 65. And so by moving down one and across five in the chart, we’ve added 15 to 50.

And the answer is that there are 65 people going to the party.

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