You know you have to divide, but you can't decide which way.
Use your units. The answer will be (money) per (can of soup), so
you know you have to divide the money by the number of cans.
(If you divided (cans of soup) by (money) then the answer you get
would be (cans) per (dollar). That's not what you need.)
(2.50) / (4 cans)
= (2.50/4) per can
= 0.625 per can .