Okay so is your question like how many plain and graphic he needs to sell to get more than $1200??
For example if he sells 100 plain shirts(which will come up to $500) and 100 graphic shirts($700) the Total amount earned would be $1200, so we would need to sell at least 100 shirts of each or more to get to $1200.
Also if he sells 240 plain shirts he will get $1200
Or if he sells 172 graphic shirts he will get $1204.