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.

tell me what i'm doing wrong?