Hi,
This is a rare question.
Its feature is “if … if”.
It appear since Primary 3. Something like if A give 10 to B, they will be equal. If B give 10 to A, they will be in certain ratio.
It includes After – Before and Before – After scenario.
We shall start from the first if.
If 46.5 litre of water is drained from Tank X, the volume of the water in Tank X will be 3/8 of the total amount of water in Tank X and Tank Y.
This means that at After X : Y = 3 : 5.
We shall reverse the pouring to Before.

Tank X 
Tank Y 
Total 
After 
3u 
5u 
8u 
Before 
3u + 46.5 
5u 





Then we look at the second if.
If 35.2 litre of water is drained out from Tank Y, the volume of the water in Tank Y will be 15% less than that of the water in Tank X.

Tank X 
Tank Y 
Total 
After 
3u 
5u 
8u 
Before 
3u + 46.5 
5u 
8u + 46.5 
After 
3u + 46.5 
5u – 35.2 
Ratio 
100%

85% 

20

17 
(3u + 46.5) / 20 = (5u – 35.2) / 17
17 x ( 3u + 46.5) = 20 * (5u – 35.2) (Cross multiplication)
51u + 790.5 = 100u – 704
790.5 + 704 = 100u – 51u
49u = 1494.5
1u = 30.5
8u = 244
244 + 46.5 = 290.50
Other same category question include
If A sells 100 cakes and B sells 200 cakes a day, A will have 300 cakes left when B sold all the cakes. If A sells 200 cakes and B sells 100 cakes, A will have 500 cakes left when B sold all the cakes. (Number are for example purpose)