Solve this problem on paper using all four steps.

A girl scout troop sold cookies. If the girls sold 5 more boxes the second week than they did the first, and if they doubled the sales of the second week for the third week to sell a total of 431 boxes of cookies, how many did they sell each week?





So I'm supposed to figure this out using a single variable. Since the only week that isn't based on another week number is week one. I'm using it as X (variable). So week 1 is x, week 2 is x + 5, and week three is 2x + 5. So if I'm correct the equation to solve it should be x + x + 5 + 2x + 5 = 431. After simplifying it comes to 4x + 10 + 431. Then you subtract 10 from both sides and multiply both sides by the reciprocal of 4 to get the let side to only have x. which I subtract the 10 and get 4x = 421. After multiplying by the reciprocal of 4 (1/4?) however. I get a decimal. And I'm pretty sure you can't sell a quarter of a box of cookies.

Please tell me what I'm doing wrong?