Answer
The equivalent rate when she earns $240 is 12 hours
Explanation
She earns $80 for every 4 hours she teaches a tennis lesson
Using this relationship
cost = rate x time
cost = $80
time = 4 hours
Now, we want to calculate the amount she earned per hour when she teaches a tennis lesson
80 = rate x 4
rate = 80/4
Rate = $20/hour
This implies that she earns $20 for every lesson she teaches one hour
If she earned $240 then the equivalent rate can be calculated as
Cost = rate x time
cost = $240
rate = $20/h
240 = 20 x time
time = 240 / 20
12 hours
Therefore, the equivalent rate when she earns $240 is 12 hours