Hello!
First you have to find how much he paid for the brushes
You can do this by multiplying the amount of brushes he bought by the price of each brush
50 * 3 = 150
He spent $150 on the brushes
Next you have to find out how much he made selling them
You can find this by multiplying the amount of brushes by the amount they sold for
5 * 50 = 250
Next you subtract the amount he earned by the amount he spent to get the amount he made
250 - 150 = 100
He made $100
Hope this helps!