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:
P(X<340)
Z= 340-350/10.25=-0.98

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...oblem is: P(Z<-0.9756)
And because Z is normal probability, it is symmetrical on both sides of zero. Then: ...