Answer:
4 pints
Explanation:
The conversion from pint to ounce is:
1 pint = 16 ounces.
The teacher collected 19 pints, that is, 19*16 = 304 ounces of lake water.
If there are 60 students, and each of them use 4 ounces, the total of lake water used is 60*4 = 240 ounces.
So, the amount of lake water left is 304-240 = 64 ounces, that is, 64/16 = 4 pints.