Question

suppose you flip a coin n times, and the probability of getting heads 15 times is .0148. What's n?
A. 19
B. 20
C. 16
D. 17
E. 24
I don't know how you would work backwards to find n help

Answer 1

use binomial theorem
\[\left(\begin{matrix}n \\ 15\end{matrix}\right) (\frac{1}{2})^{15} (\frac{1}{2})^{n-15} = 0.0148\]
which reduces to
\[\left(\begin{matrix}n \\ 15\end{matrix}\right) (\frac{1}{2})^n = .0148\]
this cannot be solved algebraically, but since its multiple choice just plug in each option for "n"

Answer 2

oh okay so you would divide n by 15 right and then multipy by the 1/2^n? @dumbcow

Answer 3

im also getting all these numbers like 1.2768352e-6? for everyone am i doing something wrong?

Answer 4

no its combinations not division'
\[\left(\begin{matrix}n \\ 15\end{matrix}\right) = \frac{n!}{(n-15)! 15!}\]

Answer 5

okay but for some reason im still getting everything to the e power so i think im doing something wrong.