On the first day, the lizard ate 50% of the 10 crickets it was given. 50% of 10 is 5 (10 times 0.5), so five crickets remain. The next day, the lizard ate 20% of the 5 remaining crickets. 20% of 5 is 1 (5 times 0.2), and 5-1=4, so 4 crickets are left.