Mary left Kim's house and drove toward the lake at an average speed of 50 mph. Chelsea left at the same time and drove in the opposite direction with an average speed of 65 mph. Find the number of hours Chelsea needs to drive before they are 345 mi. apart.

Chelsea needs 3 hours to drive before they are 345 mi. apart.

Step-by-step explanation:

Mary drove towards the lake with an average speed of 50 mph.

Chelsea left at the same time and drove with an average speed of 65 mph but in the opposite direction.

We have to calculate number of hours Chelsea needs to drive before they are 345 mi apart.

In this question we assume the time taken by both Chelsea and Kim be t to reach 345 mi. apart.

Then distance traveled by Kim in t hours d = Speed * t = 50*t

Distance traveled by Chelsea in t hours d' = Speed * t = 65*t

Total distance covered by both = 345 mi.

So total distance = d + d' = 345

50t + 65t = 345

115t = 345

t = 345 : 115 = 3 hours