1: 2 = 4: 8 the product of both the means and the extremes is
2
4
8
16

Lets say you have x:y=z:t
the means are y and z
the extremes are x and t
we have
1:2=4:8
means=2 and 4
extrememes=1 and 8
product of means=2 times 4=8
the product of extremes=1 times 8=8

the answer is 8