Math一問~~~~~

let the number of DCDs on the shelf is x .
Then The number of VCDs is x+10
The number of CDs is x+10+4=x+14
a disc is selected at random , the probability of selecting a VCD is 10/27
So
(x+10)/(x+x+10+x+14)=10/27
(x+10)/(3x+24)=10/27
30x+240=27x+270
3x=30
x=10
the number of VCDs on the shelf is 20