Grades 1-2 Video Solutions 2021-video 2021 1-2/16

Video Transcription

Problem number 16. The kangaroo had two branches for lunch. Each branch had 10 leaves. The kangaroo ate some leaves from one branch. Then from the second branch,  it ate as many leaves as were left on the first branch. How many leaves in total were left on the two branches? A, five, B, six, C, eight, D, 10, or E, 15.  Notice that this problem does not tell you exactly  how many leaves it ate from one branch. That means that instead of trying to add up some numbers, we probably need to find a pattern that will help us solve this problem.  As always, it's good to start with a picture.  There are 10 leaves on one branch and 10 leaves on the other  for a total of 20 leaves.  Now let's look for our pattern. The problem says that the kangaroo ate some leaves from one branch.  So let's start by thinking that maybe the kangaroo ate two leaves from the first branch.  It says that then from the second branch, it ate as many leaves as were left on the first branch.  So if the kangaroo ate two leaves from the first branch, there were eight leaves left.  So it would have eaten two leaves from the first branch and eight leaves from the second branch.  If that's the case, it ate 10 leaves  and the problem is asking how many leaves were left. If it ate 10 leaves total, then 20, which is the total number of leaves on both branches minus 10, leaves us with 10 leaves.  Now let's try a different number to start.  What if the kangaroo ate three leaves from the first branch? One, two, three.  Then there were seven leaves left, which means it ate seven leaves from the second branch.  Three plus seven is 10. And again, 20 minus 10 leaves us with 10 leaves.  What if it ate six leaves from the first branch? One, two, three, four, five, six.  Ate six, there were four left. So it ate four from the second branch. Six plus four is 10, 20 minus 10 is 10.  So it seems like the answer will be 10.  By now you might've noticed that the reason the number of leaves eaten always is 10 is because you start with 10 leaves on the first branch.  Whatever you have left is exactly the same what the kangaroo will eat from the second branch. So it will always add up to 10. So we can keep trying it.  If the kangaroo ate all 10 leaves from the first branch and there were none left, then it would not eat anything from the second branch. So there would still be 10 leaves left.  So no matter what, the answer is going to be 10.  The kangaroo will eat 10 leaves. So there will be 10 leaves left.

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