In a survey it was found that 21 people liked product A, 26 liked product B and 29 liked product C. If 14 people liked products A and B, 12 people liked products C and A, 14 people liked products B and C and 8 liked all the three products. Find how many liked product C only

Let A, B, and C be the set of people who like product A, product B, and product C respectively.
Accordingly, n(A)=21, n(B)=26, n(C)=29, n(A∩B)=14, n(C∩A)=12, n(B∩C)=14, n(A∩B∩C)=8
The Venn diagram for the given problem can be drawn as above.
It can be seen that the number of people who like product C only is 29−(4+8+6)=11