Determining the probability using binomial distribution.
The Bell telephone co. surveyed an apartment building with 500 units to find out who subscribes to their service. Turns out 70% of the units use Bell's service.
Find the probability that less than 340 units are using Bell services so we are looking for:
...oblem is: P(Z<-0.9756)
And because Z is normal probability, it is symmetrical on both sides of zero. Then: ...