Answer
The number of elements belongs exactly one set = 21.
Work Step by Step
From the given Venn diagram,
the region I represents the number of elements belongs to only set A.
The region III represents the number of elements belongs to only set B.
The region VII represents the number of elements belongs to only set C.
It's given from the Venn diagram that,
The number of elements belongs to region I = 5
The number of elements belongs to region III = 6
The number of elements belongs to region VII = 10
Therefor The number of elements belongs exactly one set = 5 + 6 + 10 = 21.
