## Jovanmommy_13 2 years ago What is the probability that out of 125 babies born, at least 70 will be girls? Assume that boys and girls are equally probable, and round your answer to the nearest tenth of a percent. A.99.0% B.70.4% C.10.5% D.51.2%

1. FeoNeo33

10 20 30 40 50 60 70 / 80 90 100 110 120 ...125 70 / 50... Which I pick... B.

2. FeoNeo33

By the way, I made up that way to get the answer, not a real technique.

3. amistre64

im thinking it sounds like a binomial probability setup

4. Jovanmommy_13

that what these are

5. amistre64

125 C 70 times success^(70) times failure^(125-70) 125 C 70 (1/2)^(70) (1/2)^(125-70) soo:$\sum_{k=70}^{125}\binom{125}{k}.5^{k}.5^{125-k}$

6. amistre64

or we can try a normal approximation :)

7. amistre64

mean = np = 125(.5) = 62.5 so we would expect 62.5 of the population to be girls sd = sqrt(npq) = sqrt(62.5*.5) = sqrt(31.25)

8. amistre64

do you have to do this all by hand or do you have a ti83?

9. amistre64

$z=\frac{x-mean}{sd}$ $z=\frac{70-62.5}{\sqrt{31.25}}=1.34$ looking on a ztable; that corresponds to about 100% - 91%

10. amistre64

so yeah, it aint gonna be B, but it does look like it will be close to 10%