Answer :

In 1 cl there are 10 ml

So in 2 centilitres there will be 20 millilitres  
always when moving from centiliters to milliliters, you multiply by 10.
Which mean 1 centiliter = 1 * 10 milliliter = 10 milliliter 
So we get:
2 centiliters = 2 * 10 milliliters = 20 milliliters. 

Which means Jason takes 20 milliliters of Medicine.

Hope this Helps :)