The school store opened on the first day of school with 35 notebooks and 15 pencils. Within two days it sold all of these items. On the first​ day, twice as many notebooks were sold as pencils. On the second​ day, for every 5 notebooks​ sold, 2 pencils were sold. How many notebooks and how many pencils were sold on each​ day?

first day:

number of pencils sold --- x
number of notebooks sold --- 2x
so there are 35-2x notebooks left and 15-x pencils

On the second​ day, for every 5 notebooks​ sold, 2 pencils were sold
---> 35-2x : 15-x = 5 : 2
70 - 4x = 75 - 5x
x = 5

first day sales: 5 pencils and 10 notebooks
2nd sales: 35-2x or 25 notebooks and 15-x or 10 pencils

notice 25: 10 = 5 : 2