A) Convert all Large box into small box ratios,
And focus on small boxes for painting:

12 large / 2 large = 6 units

2 large Box : 5 small box
= 12 Large Box : 30 small box (multiply by 6)

So 12 large is same as 30 small,
So 11 small + 12 large
= 11 small + 30 small
= 41 small boxes

41 small => 82 L paint
1 small => 2L paint //

B) remaining box:
Remaining Small = 34 - 11
= 23 small boxes

Remaining Large = 15 - 12
= 3 Large boxes
= 3 * 5 / 2 (ratio 2:5)
= 7.5 small boxes

Total remaining = 23 + 7.5
= 30.5 small boxes

Remaining paint = 30.5 * 2L
= 61L of paint //