About this video
A tank starts with 200 litres and leaks 15 litres an hour for 6 hours. The water lost is 15 × 6 = 90 litres, so the amount left is 200 - (15 × 6) = 200 - 90 = 110 litres.
You'll be able to
- write one expression for a 'start, then lose at a rate' word problem.
- use order of operations (multiply before subtract) to evaluate it.
- multiply 15 × 6 by splitting it into 10 × 6 and 5 × 6.
Key words
- order of operations
- The rule that you multiply and divide before you add and subtract (after anything in parentheses).
- litres per hour
- A rate telling how many litres change each hour.
Transcript
- [Instructor] We're told a water tank has 200 liters of water. Due to a leak, the tank lost 15 liters of water an hour for six hours. How much water was left in the tank after the six hours? Pause this video and try to work through that and then we will do this together.
All right, so let's see what's going on here. So we're starting, it says the water tank has 200 liters of water. That's what we're starting out with. And I'll just assume all these numbers are in liters. Due to a leak, the tank lost 15 liters of water an hour for six hours.
So that lost tells us that we're going to be left with less water than we started with, that we're going to be subtracting from our original 200 liters. And let's think about how much we are subtracting. It's losing 15 liters of water an hour for six hours. So one way to think about it is that's 15 liters per hour times six hours.
This is how much is going to be lost. So we could start with our 200 liters and we're gonna subtract 15 times 6, 15 times 6. Now, order of operations would tell us to do the multiplication first regardless, but just to be extra careful, I'll put those parentheses there.
So now let's do it. What is 15 times 6? We could work it out on the side over here. Many of y'all might be able to do this in your head. You could say, hey, 10 times 6 is 60 and 5 times 6 is 30, so this is going to be 90. Or you could say, 5 times 6 is 30, regroup that 3 or carry the 3, however you wanna describe it, and then 1 times 6, this is really a 10.
1 times 6 is 6, plus 3 is 9, but we're of course in the tens place so that is going to be 90. So this right over here is going to be 90. So it all boils down to 200 minus 90, which is going to be equal to 110, and we're done. That's how much water is left in the tank after six hours.
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