If in a class of 30 students10 take tea but not coffee and 14 take tea then how many take coffee but not tea?

Unless we can assume that all students take tea, or coffee, or both, then we do not have enough information in the facts given.

All we know competely is the details regarding tea.

14 take tea and 10 of those do not take coffee.

This tells us that 4 people who take tea do also take coffee.

But, this leaves 16 people who definitely don't take tea .... these people 'might' take coffee, but perhaps some of them only take water, or cola, or whatever.