Writing at its finest. Subtext! The scene shows a kid asking for help on a math problem. However, what the scene REALLY shows is how the world of “The Wire” is. It also advances the plot further (Wallace and his developing drug problem, and his slacking at work as a drug dealer himself).
A word problem about a bus picking passengers up totally confuses and stumps the kid, but a word problem given by his brother Wallace about drugs comes easy to the kid because he is scared for his life. The kid doesn’t seem older than 10, and he already knows how to sell drugs. Who tells a little kid “… Imma fuck you up”?
The writers didn’t need to show people running from gun shots or people getting killed. There were no stunts or special effects used. All they used was a kid, his older brother, and a math book and they created something much more powerful where 95% of the action happens in our imagination.