setting values and percentages

[EarlyBird2]

however
if you have a word var say numx
and x =4095
then 50*numx/100 the result is 81
the word var causes a intermediate stage overflow (bits 0-15 of 204750 =8142)

I do not understand this.
What is the relationship between numx and x?
Why does an intermediate stage overflow occur and what is an intermediate stage overflow?