From the concluding part of the question, it may be inferred that If he picked strawberry for an hour that means he picked strawberry for 60 minutes.
Okay so for 15 minutes he earned $2
Then in 60 minutes he will earn X
So 15 * X = 2 * 60
Then it follows that X = 120/15 = $8
On the other hand if he earns $2.40 for 15 minutes picking blueberry
Then it follows that he earned $X in 45 minutes.
Also X * 15 = 2.4 * 45
X = (2.4 * 45) / 15 = $7.2
So he earned $7.2 + $8 = $15.2
So
