Answer
See below.
Work Step by Step
a) $(4(1)+8)/2−4=2$
$(4(2)+8)/2−4=4$
$(4(3)+8)/2−4=6$
$(4(4)+8)/2−4=8$
$(4(5)+8)/2−4=10$
Conjecture: the result is the original number's double.
b) Proof: for any $n$, the procedure is as follows:
$n$, then $4n$, then $4n+8$, then $(4n+8)/2=2n+4$, then $(2n+4)−4=2n$, thus we always get back the original number's doible.