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Let R be the relation from A = {2,3,4,5} to B = {3,6,7,10} given by R = {(a,b): a divides b, a in A, b in B}. The number of elements in R⁻¹ is:

1. 0
2. 3
3. 4
4. 5

Correct answer: 5

Solution

The divisibility pairs are (2,6),(2,10),(3,3),(3,6),(5,10) - five pairs - and R⁻¹ contains exactly the reversed pairs, so it also has 5 elements.

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