It is given that Motorists A, B and C are travelling from Town X to Town Y which is 36 km apart.
It is given that when Motorist A is reached town Y , motorist B need to cover 12 km distance to reach town Y and motorist C need to cover 15 km distance to reach town Y.
A can travel 36 km in one hour.
B can travel 24 km in one hour.
C can travel 21 km in one hour.
Now, B can reach to point Y in half hour as there is only 12 km left to cover.
So, in half the C covers only 10.5 km.
And initially it is 15 km apart from point Y but when B reached to point Y.
C left with 4.5 km that is
![15-10.5=4.5\operatorname{km}]()
Hence Motorist C from Town Y when Motorist B reached Town Y is 4.5 km.