The weighted number of exposures $($WE$)$ of an advertising message over a given medium is given by:

a$)$ WE $=$ reach $*$ frequency.

b$)$ WE $=($reach $*$ frequency$)/$ impact.

c$)$ WE $=$ reach $*$ frequency $*$ impact.

d$)$ WE $=($reach $+$ frequency$)/$ impact.