Answer:
30 hours.
Step-by-step explanation:
Richard can build 15 snowballs in 1 hour
But 2 snowballs melt every 15 minutes
In 1 hour, the number of snowballs that will have melted is:
× 2 = 8
The number of snowballs that will have remained = 15 - 8 = 7
So 7 snowballs will have remained in 1 hour
For 210 snowballs to have remained, it will take:
× 1 = 30 hours.