Question 984508
The first mixing made  30 parts of milk and 120 parts water, a total of 120+30=150 parts.  Ratio in tank Q is  30:120, as milk to water.
Tank P now has  90 parts total, of just milk.


The second mixing  took out 30 parts from Q, and put into P.  
90 parts of milk + {{{30(3/12)}}} parts of milk, and  {{{120-(90+(1/4)30)}}} parts water.


At this stage, you should summarize exactly what you have in tank P and in tank Q; before going on to the two next mixings.  I leave this unfinished intentionally.