Discrete Mathematics
# Bijections

A function $f$ maps the elements of $A = \{14, 16, 18, 20\}$ to elements of $B =\{55, 66, 77, 88, 99\}.$ How many of the possible maps $f$ are not injective?

**Details and assumptions**

A function is **injective** if each element in the codomain is mapped onto by at most one element in the domain.