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# probability

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Two different 2-digit numbers are randomly chosen and multiplied together. What is the probability that the resulting product has a units digit of 1?

Dec 14, 2020

#1
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There are: 90 nCr 2 =4005 possible products.

Only 153 of them end in "1"

Therefore, the probability = 153 / 4005 = 17 / 445

Dec 14, 2020
#2
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So the probability will be  4/100 = 0.04     (I have fixed a typo)  Thanks guest

Dec 14, 2020
edited by Melody  Dec 14, 2020
#4
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A typo: 4 / 100 = 1 / 25

Guest Dec 14, 2020
#3
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Hi Melody: I think they want "Two different 2-digit numbers are randomly chosen and multiplied together."

Dec 14, 2020
#5
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Hi guest,

My original answer (the one you referred to) does look at the multiplication of 2  two digit numbers

The header row is the units digit of the first number

The header column is the units digit of the second number.

What I did not take into consideration is that the numbers are supposed to be different from one another.

This makes the question much more difficult.  Each cell will no longer occur an equal number of times.

So I have redone it.  I think this method is right but I am less confident than i was with the original answer.

Melody  Dec 14, 2020