Imagine a chest of drawers where the top drawer contains 10 boxes.
This is the 100s drawer and each box contains 10 other smaller boxes. These smaller boxes are usually kept closed.
In the next drawer, the 10s drawer, we have 10 of the smaller boxes. In each of these boxes we have 10 counters.
In the bottom drawer we have 10 counters.
To do the subtraction we're going to represent the combined weight of the giraffe and her calf by taking out one big box from the top drawer, leaving 9 representing 900.
We empty the next drawer completely (no 10s) and we leave 4 counters in the bottom drawer.
The chest of drawers now represents the number 904.
Now we're going to take away 829, the mother giraffe's weight.
We remove 8 big boxes from the top drawer leaving just one.
When we try to take away 2 small boxes from the 10s drawer we find it's empty! But we have 1 big box in the top drawer, the 100s drawer, so we take the box and open it and fill the 10s drawer with the 10 small boxes. We can now take away 2 of these leaving 8.
We need to take 9 counters away from the bottom drawer, but it only has 4 counters.
Go to the middle drawer and take a small box of counters, leaving 7 small boxes.
Open the small box and pour the 10 counters into the bottom drawer making 14 altogether.
Now we can take away 9 counters leaving 5.
Let's open all the drawers and see what we have.
The top drawer is empty, the middle drawer, the 10s drawer, has 7 small boxes, and the bottom drawer has 5 counters. So the chest of drawers represents the number 75.
The calf weighs 75.