The difference between a hundred and a number is four times that number

20

Step-by-step explanation:

We know that the difference between a hundred and a number is four times that number.

So assuming the number to be x, we can write the difference between a hundred and a number as 100 - x and four times that number as 4x.

So by combining these together, we get:

100 - x = 4x

100 = 5x

x = 100/5

x = 20

Therefore, the unknown number is 20.