Twelve round fish swim across the page, each carrying a subtraction problem like 14 − 5 or 20 − 8. The child solves the problem and colors the fish using the key at the top: 9 is orange, 10 is blue, 11 is purple and 12 is green.
Every answer lands on one of those four numbers, three fish each, which gives the page a quiet self-check. A fish that comes out as 8 or 13 does not have a crayon, and that is the signal to count again.
It is a good way to get a lot of within-20 practice done without it feeling like a drill, and the black and white fish are big enough for young hands to color neatly.