R = (2s - 6t + 5)/2
2R = 2s - 6t + 5
2s = 2R + 6t - 5
s = (2R + 6t - 5)/2
Try this solution, note, the shown way is not the shortest one.
The answer is marked with red colour.
The amount of liters is 200,if the orange juice is x than grape juice. 200-x
45x+35(200-x)=200*40
45x+7000-35x=8000
10x=1000
x=100
8 books with 2 sheets each and 1 extra sheet as leftover