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### Course: Class 2 > Unit 3

Lesson 4: Word problems within 100# Subtraction word problem: tennis balls

Sal solves a 2-step subtraction word problem with numbers less than 100.

## Want to join the conversation?

- How can I tell if I am adding or subtracting?(0 votes)
- There are universal symbols for addition and subtraction.

When you see this sign (+) then you know to add

When you see this sign (-) then you know to subtract(0 votes)

- How do you subtract a smaller number to a bigger number?(0 votes)
- The answer goes less than zero, and goes into the negatives, such as 5-7= -2, because it goes two spots below zero.(6 votes)

- How much is 700 penys(1 vote)
- Because 100¢ (one hundred pennies) are equal to $1 (one dollar). We can divide 700 by 100 (700/100) to get a product of '7' and so we can evaluate that 700 pennies are equal to 7 dollars ($7).

Hope this helps!(1 vote)

## Video transcript

- Mrs. Grange's second grade
class has 45 tennis balls. Whenever I do a word problem like this I just like to write down the information that they're giving me. So we're gonna start with 45 tennis balls. They then tell us that 19 tennis balls, so she starts with 45 tennis
balls to play with at recess. They then tell us that 19 tennis balls get thrown over the fence and 6 tennis balls just disappear. How many tennis balls does
Mrs. Grange's class have left? So what I'd like to do, when I do these type of word
problems, let's say, okay, first of all where am I starting from? Well, I'm starting from 45 tennis balls and then whats happening? Am I gaining tennis balls
or am I losing tennis balls? Well I started with 45 or Mrs. Grange's class started with 45 then 19 tennis balls get
thrown over the fence so they're losing the balls. And then another 6 tennis balls disappear, so both of these are losing tennis balls. And since I'm losing something, since I'm losing the tennis balls, or some of the tennis
balls that I started with I should subtract here. Another clue that subtraction
might be interesting is because it says how many tennis balls does Mrs. Grange's class have
left, which implies that, well, she doesn't have
all the tennis balls that she started with so we're probably going to subtract some balls. But it comes straight
out of the word problem. 19 tennis balls get thrown
over the fence, she loses them, so we should subtract that
from the original amount. And then another 6 just
disappear, so we should then subtract that on top of
the 19, so let's do that. So let's first subtract the 19. So we start with 45 tennis balls, and then 19 balls get
thrown over the fence, so 19 get thrown over the fence
so we're subtracting them. They lose those balls, so
what is this going to be? Well if we look in the ones place we have 5 ones minus 9 ones. Well, since 5 is less than 9
we want to do some regrouping. So let's go to the tens place. Instead of 4 tens, lets
try that as 3 tens, and then take that 10 , turn it into ones so it's gonna be 10 ones. 10 ones plus 5 ones is 15. Now we're ready to subtract. 15 minus 9, well that's gonna
be 6 because 9 plus 6 is 15. And then 3 tens minus 1 ten
is going to be 2 tens, so 26. But are we done? No. This is just the number
of balls we have left after the 19 get thrown over the fence but before we count the
6 that just disappeared. So we also have to take away 6 for the 6 balls that disappeared. So let's do that. So after 19, so we started with 45, 19 get thrown over the fence,
we have 26 at that point. Then, 6 balls just disappear
so we have, once again they disappear, they're not,
you know, magically appearing. Since they're disappearing we subtract. So we, they're disappearing,
so we want to subtract those. So how many do we have left? Well, 6 ones minus 6 ones is 0 ones. 2 tens minus, well nothing here, that's just going to be 2 tens. So how many do we have left? Well, or how many does Mrs. Grange's second grade class have left? Well you see it right here. They have 20 tennis balls,
20 tennis balls left. Started with 45, 19 get
thrown over the fence so they lose them,
that's why we subtracted. That gets us to 26,
then 6 of them disappear so we take the 6 away
from the 26 to get to 20.