It takes 39 minutes for a car to be cleaned at a carwash. How many minutes does it take to clean 40 cars?
What information are we starting with? We know it takes 39 minutes to clean one car and we need to do that 40 times: 39 minutes for one plus 39 for the second car. Adding the 39 to itself 40 times will take a very long time. There has to be a better way.
The better way would be to multiply. Multiplication is repeated addition. This means instead of adding 39 to itself 40 times, we can take the 39 minutes it takes for one car to be cleaned and multiply that by the 40 cars we need to wash. Starting all the way on the right, we multiply zero times nine equals zero. Then, we move to the tens’ place and we multiply zero times three — zero.
Now, we’ll be multiplying with the four in the tens’ place times the nine in the ones’ place. Four times nine equals 36. We write down our six and we carry our three. From there, we’ll multiply four times three which equals 12. And then, we will add the additional three that we have already carried over. Four times three is 12 plus three is 15. And we’ll write down our 15.
Now, we need to add. Starting all the way on the right, zero plus zero equals zero, zero plus six equals six, five plus zero equals five, one plus zero equals one.
Now that we finish multiplying, we have to think about what does this mean: 1560. Well, we multiplied 39 minutes per car times 40 cars that we need to clean. This 1560 represents the number of minutes it would take to clean 40 cars. Our final answer is then 1560 minutes.