It will never be empty.
Speed of filling by A and B pipes is greater than speed of pipe c to empty the cistern.
Further pipes A and B are opened before pipe C.

368 units of water are added in 864 units of M.
864 units of M contain 120 units of A, 616 units of B and 128 units of C .
As concentration of B is 50% in final mixture, volume/wt of total mixture is 2*616=1232 units.
Hence 1232-864 = 368 units of water are added to 864 units of M.

368 units of water are added in 864 units of M.
864 units of M contain 120 units of A, 616 units of B and 128 units of C .
As concentration of B is 50% in final mixture, volume/wt of total mixture is 2*616=1232 units.
Hence 1232-864 = 368 units of water are added to 864 units of M.

864 units of M contains chemical X ( 480 units) and chemical y ( 384) as their ratio is 5:4 in M.
480 units of X contain 120 units of A and 360 units of B ( ratio 1:3).
384 units of Y contain 256 units of B and 128 units of C.
so 864 units of M contain 616 ( 360+256) units of B,
864 units of M contain 120 units of A, 616 units of B and 128 units of C.

38.4 litres of pure milk is left in final quality/quantity of mixture.

12 years ago
explanation: let k years ago
then (4x+k)/(3x+k)=5/3
then x=-2k/3
now present ages become -8k/3 and -2k
according to the given statement i.e, after 4 years this ratio will become 9:7
the ratio becomes
(4-(8k/3))/(4-2k)=9/7
by solving
k=-12
i.e, 12 years ago