In math we learned subtraction with regrouping over zero. It's harder than regular regrouping subtraction. But it's still fun. Here is an example:
Now I can teach you how:
First, you have to know what numbers you are using. Your numbers are 901 and 378. Now, you have to go to the tens place because you can't subtract 8 from 1. But there is a problem, there's no number in the tens place. So you have to go to the hundreds place. Take one hundred from 900. You get 800. Put the one (from the hundred) in the tens place. Now you have ten tens. But you have to give 1 ten to the ones place. So, the answer is 523.