Get some marbles or any other small objects and count them to make sure they are 20 in number. Put them into a container, and start picking out one after the other till you have four in hand. Place those four somewhere apart. Do this repeatedly until you have no marbles left in the container, and all of them are in groups of four, all placed apart from each other.
Count the number of groups that you have. You should have five groups in total since you had divided the original number of marbles by four. You used repeated subtraction in this process by removing, or subtracting, four marbles at a time from the original group of 20.
Repeat the same procedure with the same number of marbles, but this time repeatedly subtract only three marbles at a time from the original group. You will end up with seven groups, six of them having three marbles each and the seventh with only two. This means that three can be subtracted repeatedly six times from 20, and two will be the remainder. Put another way, 20 divided by three gives six with a remainder of two.
Redo the first calculation, this time using numbers. That is: 20 -- 4 -- 4 -- 4 -- 4 -- 4 =? Rationally, the answer to this statement is zero. This means that four is a perfect divisor for 20. Count the number of fours in the statement to discover that they are five in number, meaning 20 divided by four gives five as the answer through repeated subtraction
Repeat the second calculation using numbers as well. That is: 20 -- 3 -- 3 -- 3 -- 3 -- 3 -- 3 =? Here, the answer is not zero but two. This shows that three is not a perfect divisor of 20, since 20 divided by three gives six with two as the remainder through repeated subtraction.