2021 JMPSC Invitationals Problems/Problem 15
Contents
[hide]Problem
Abhishek is choosing positive integer factors of with replacement. After a minute passes, he chooses a random factor and writes it down. Abhishek repeats this process until the first time the product of all numbers written down is a perfect square. Find the expected number of minutes it takes for him to stop.
Solution
Note that , so we want to "select" for the numbers that are not factors of
. This is
of them. In addition, for the factors of
, exactly
of them are perfect squares, and for each turn, there is a
probability that the product will be a perfect square since the product of the numbers before does not affect the probability (in this case). Therefore, for each turn, there is a
probability Abhishek will end, which means he is expected to end on his
th turn.
~Geometry285
Solution 2
It factors as , and say that the form of the product of the numbers is
. With equal probability,
is even and
is odd at every step since at the first step,
appears with
probability and
appears with
probability. The same applies to 47, or
. The same goes for
. Because the numbers of
range from 0 to 2021 each time, then obviously half of those numbers are odd and half are even. The probability of each
being even (which is the requirement for being a perfect square) is
, so the answer should be
.
~lamphead
See also
- Other 2021 JMPSC Invitationals Problems
- 2021 JMPSC Invitationals Answer Key
- All JMPSC Problems and Solutions
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