Solving two-step word problems involving adding and subtracting decimals | Khan Academy

We are told it takes Ally a total of 51.84 KM to get to work. She travels 6.07 km by car, 1.3 km by walking, and the rest by train. How many kilometers is Ally's train ride? Pause the video, have a go at it before we do it together.

Okay, so if we were to add the train ride plus the walk plus the car, it should total up to 51.84 km. Now we know what the walk is and what the car ride is; we just need to figure out the train ride.

Another way to think about it is the train ride should be equal to the total 51.84 minus the amount that we walk minus the amount that we drive by car. The amount that we walk is 1.3 km, and the amount that we go by car is 6.07 kilometers. So we just need to figure out what this is.

Let's do that. First, let's subtract the 1.3 and then let's subtract the 6.07. If I first try to calculate this part right over here, I have 51.84 minus—make sure to line up the place value—1.3.

All right, 400 minus nothing, minus 0 hundreds is 4 hundreds. 8 hundreds—make sure I pronounce it right—8/10 minus 3/10 is 5/10, and then 1 minus 1 is 0, and then 5 minus 0 is 5. So this right over here is 50.54 km now.

Let's subtract 6.07 from that, and I could actually just do that right over here. So let's say minus 6.07. All right, so we go to the hundred place; we can't subtract 7 from 4—a little bit of regrouping. Let's take a tenth, so now we only have four tenths here.

A tenth is the equivalent of 10 hundredths, so now we have 14 hundredths. We could subtract 14 minus 7 is 7, 4 minus 0 is 4. We have our decimal here, and we could do grouping again.

Or you might just recognize that 50 minus 6 is going to be 44. If we wanted to regroup, we could say, “Hey, let's take a 10 here,” and then that's 10 ones. 10 minus 6 is 4, and then you bring down those four tenths.

But either way, the amount that Ally travels by train is 44.47 km.