Each worm costs $0.16.
When you multiply $0.16 (the cost of each individual worm) x 5, you get 0.80 per group of every 5 worms.
When you multiply 0.80 x 5 (the number of groups with 5 worms in them each; 5x5 = 25 worms total), you get $4--which is the total amount of money you spent on all 25 of the worms according to the word problem outlined above.
I hope this helps! :)