Method 1 :
amount of paint at first ——- 12u
(1/4) x 12u —– 3u (kitchen)
(1/3) x 12u ——- 4u (living room)
12u – 3u – 4u = 5u ——- 10l
1u ——- 10/2 = 2l
12u ——- 12 x 2 = 24l

Method 2 :
1 – 1/4 – 1/3 = (12 – 3 – 4)/12 = 5/12
5/12 ——- 10l
1/12 ——- 10/5 = 2l
12/12 ——- 2 x 12 = 24l

Ans : 24l.

Thanks. My child say method 2 much easier to understand.

Don’t mention. I believe that generally students should, if not must, explore the various methods  in solving Math questions as this would put them in good stead in facing more difficult ones in future.

¼ + 1/3 = 3/12 + 4/12 = 7/12

After using 7/12, he had (1 – 7/12) = 5/12 left

5/12 = 10

12/12 = 10 × 12/5 = 24 litre at first