Part 1
The snow melts at 2.5 inches per hour. After 2 hours, 2*2.5 = 5 inches has melted overall. We started with 15 inches, so there are 15-5 = 10 inches of snow left after 2 hours.
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Part 2
Let t be the number of hours that have elapsed. Because the snow melts at a rate of 2.5 inches per hour, this means after t hours, 2.5t inches of snow melts away. Subtract this from the 15 inches we start with to get 15-2.5t. This expression is the same as -2.5t+15