There are crows and groundhogs in a field. All of them together have 15 heads and 40 legs. How many of each animal are in the field?
Let x be the number of crows and 15-x the number of groundhogs. Note that crows have 2 legs and groundhogs 4. We form an equation for the number of legs.
There are 10 crows and 15-10=5 groundhogs.