Having to do with the $1200 per month & $35 for every phone you sell
shelia made $4000 last month how many phones did she sell ?

Since her base amount is 1200, we have 1200+<number>=<number>. Since she gets 35 dollars for every phone, if the number of phones she sells is x, we have 1200+35*x=the total amount of money in a month =4000 . Subtracting 1200 from both sides, we get 2800=35*x and by dividing both sides by 35 we get x=80

There should only be 1 solution