He needs to sell 100 souvenirs
Here, we want to know the number of souveniers John has to sell
Let the number of souvenier he sold at the game be s
At a rate of $0.25 per souvenier, the amount earned on souveniers for s souveniers will be;
Now, if we added this to the amount he earns per game, we will have the total $35 earned for working at one game
Thus, mathematically;