Solving two-step word problems with decimals (adding and subtracting) | Khan Academy

We are told that Lynn has saved $80. He spent $175.00 on a gift for his brother and $229.50 on a gift for his mom. How much money does he have left after buying the gifts? Pause this video and work through this on your own before we do this together.

All right, now let's think this through. So we know how much money Lynn starts with: $80. So let me just write that down. I'm just going to assume all of this is in dollars. And then how much does he spend? Well, he spends $175.00 for his brother, and he also spends $229.50 on his mother.

So there's really two ways—and maybe more—that you could think about how to calculate how much money he has left. You could just say, okay, he starts with $80, we take away $175.00, and then we take away another $229.50.

Another way to think about it is he's starting with $80, and then let's just take away the total amount that he spends. So, what I have here in red, the $175.00 plus the $229.50, this is the total amount that Lynn is spending on gifts for his family. So what we need to do is just subtract that from the original $80 that he starts with.

So let's work through this together. First, let's add $175.00 plus $229.50. I'll start in the hundredth place: 0 + 5 is 5. Go to the tenth place: 7/10 + 9/10 is 16/10, which is the same thing as 1 and 6/10. Let me put my decimal there to make sure we have everything lined up.

Now, 1 + 7 + 2: that's 10, which is the same thing as 0, 1, and 10. Now we have 1 + 1 + 2: that's 4. So this is how much Lynn is spending. This is $404.50.

Now we just have to subtract that from the original $80. Let me write it out all the way; use that same shade of blue. Let me write it out all the way to the hundredths place, and I am going to subtract now the $404.50, the total amount that he spent on gifts.

What do we get? Well, it looks like we're going to have to do some regrouping. We can't take 5 from 0, or 4 from 0, and the first place where we could take something from is in our tens place. So let's take one of them away, so this becomes 7 tens and 10 ones.

Now, let's take one of those away. That becomes 9 ones and 10 tenths. Now let's take one of those away, so that becomes 9 tenths, and that one we took away, that extra tenth, that is 10 hundredths.

Now we are ready to subtract. 10 - 5 (hundredths) is 5. 9 - 4 (tenths): this is 5. Put our decimal there. 9 - 0 is 9; 7 - 4 is 3. And there we have it! He is left with $39.50. That's what all of this business is equal to.