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Blackishmamba
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A fair coin has 2 distinct flat sides—one of which bears the image of a face and the other of which does not—and when the coin is tossed, the probability that the coin will land faceup is ½. For certain values of M, N, p, and q, when M fair coins are tossed simultaneously, the probability is p that all M coins land faceup, and when N fair coins are tossed simultaneously, the probability is q that all N coins land faceup. Furthermore, M < N and 1/p + 1/q = 72.

In the table, select a value for M and a value for N that are jointly consistent with the given information. Make only two selections, one in each column.



OA:
M=3
N=6

Probability that all M have same outcome = 1/2^m
And probability that all N have same outcome = 1/2^n
So 1/(1/2)^m + 1/(1/2)^n = 72 thus 2^m+2^n=72
2^m(1+2^{n-m})=72=2^3*3^2... So m=3
So 1+2^{n-m} must be a multiple of 9.. so 2^{n-m}=8....
n-m=3...n=6
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Another way would be trial basis...

By solving the given equation, we can say p+q=72pq or q=p/(72p-1)

We also know p and q will have denominators in the form of 2^x

Start with option A)
If M=2, then
p= (1/2)^2 = 1/4 -> q=1/88 - not possible as denominator not of the form 2^x

Similarly, Option B)
If M=3, then
p=1/8 -> q=1/64 = 1/(2^6)
Therefore N = 6.

Answer:
M = 3 (Option B)
N = 6 (Option E)

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Quote:
Another way would be trial basis...

By solving the given equation, we can say p+q=72pq or q=p/(72p-1)

We also know p and q will have denominators in the form of 2^x

Start with option A)
If M=2, then
p= (1/2)^2 = 1/4 -> q=1/88 - not possible as denominator not of the form 2^x

Similarly, Option B)
If M=3, then
p=1/8 -> q=1/64 = 1/(2^6)
Therefore N = 6.

Answer:
M = 3 (Option B)
N = 6 (Option E)

This way of solving looks more straightforward.
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I recognized that 1/p + 1/q = 72 basically says the denominators of the probability equal 72 (because 1/FRAC = the denominator).

I then also recognized that since the probability being multiplied is 1/2, the denomenators of the probabilities will only include the prime factor of two. (2 4 8 16 32 64 etc)

I started playing with the numbers and found that 8 + 64 = 72. 8 and 64 are both numbers with only 2 as their factor. 2^3 = 8 and 2^6 = 64 therefore the answers are 2 and 6. m<n therefore 2=m 6=n.­­­­
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This is a very high quality question that tests your ability to use algebra and probability concepts jointly. do you understand how the translate a series of isolated coin flips into an algebraic expression, and how variables effect the outcome of a certain event?

a coin flip is obviously 1/2 odds of being face up, but put with 1/p and 1/q = 72, can you ascertain that P is some variable that is influenced by m? and how does it fit together with N given the available answer choices?

great question, very well written, and is essential for understanding probabilities
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