I'd normally say matter how the money works out, Sam should obviously take the electronics store job. He might learn some electronics and some business and as a result not have to spend the rest of his life cleaning buses. But here they're paying him $20 an hour to clean buses compared to $8 an hour to move electronics, so he probably should clean buses for a summer. He'll gain an appreciation for clean buses and the people who clean them. In the end Sam might help pull the cart of civilization a little farther by becoming an engineer. He'll need to know some math.
15 hours * $8 per hour = $120
$300 - $120 = $180 Sam needs to make up in sales commission
Let x be the sales price of the electronics Sam sells. We need
0.02 × x = 180
x = 180 / .02 = 50(180)= $9000
Answer: Sam needs to sell more than $9000 of electronics every week to make as much as he'd make cleaning buses.
Check: 15×8 + 9000×0.02 = 120 + 180 = 300, good