You will win eventually

Consider this game:

You have a 1% chance of winning on the first try. If you win, great!

If you don't, win on the first try, you have a 2% chance of winning on the second try.

If needed you have a 3% chance on the third try and so on until you eventually win.

What is the expected number of tries to win this game?


The answer is 12.2099606302.

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1 solution

Jeremy Galvagni
Jun 8, 2018

As I usually do with such problems, I'll start with simpler problems and look for a pattern. Maybe even a formula. Maybe a sequence in the OEIS.

Call the problem sought n=100 and I'll start with n=2: A 1/2 chance of winning and a 2/2 chance of winning if a second try is needed. E.V. = 1/2 * 1+1/2 * 2=3/2=1.5

n=3. 1st try: 1/3, 2nd try 2/3 * 2/3, 3rd try 2/3 * 1/3 * 3/3. E.V. = 1/3 * 1+4/9 * 2+4/9 * 3=17/9=.38888

n=4. 1st try: 1/4, 2nd try 3/4 * 2/4, 3rd try 3/4 * 2/4 * 3/4, 4th try 3/4 * 2/4 * 1/4 * 4/4, E.V.= 1/4 * 1+6/8 * 2 +18/64 * 3+6/64 * 4 = 71/32 = 142/64

n=5. Long story short E.V. = 1569/5^4

Ok so the denominators are n^(n-1) and the numerators, beginning the n=1 are 1,3,17,142,1569

A single hit in OEIS and the formula is there as a summation. The numbers are going to get huge but there's a link to the first 380 terms. I need the 100th.

12209960630215980300253132956815012998412714159392662223854992521128809374822896606879062793565756480077431168694642794552798934366694577253573013735421843845957043306726309232640000000000000000000000 1.22099606302 1 0 199 12209960630215980300253132956815012998412714159392662223854992521128809374822896606879062793565756480077431168694642794552798934366694577253573013735421843845957043306726309232640000000000000000000000 \approx 1.22099606302*10^{199}

So the answer is 1.22099606302 1 0 199 10 0 99 = 12.2099606302 \large\frac{1.22099606302*10^{199}}{100^{99}} = \boxed{12.2099606302}

Nice problem. I ended up calculating n = 1 100 99 ! × n 2 ( 100 n ) ! × 10 0 n \displaystyle \sum_{n=1}^{100} \dfrac{99! \times n^{2}}{(100 - n)! \times 100^{n}} , (using WolframAlpha).

Brian Charlesworth - 3 years ago

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