Dice Game Outcome
Mary and Jenna are playing a game in which each girl rolls a twenty-sided die with numbers from 1 to 20 on the faces. If the number on the die that Mary rolls is a factor of the number on the die that Jenna rolls, Mary wins. If the number on the die that Jenna rolls is a factor of the number on the die that Mary rolls, Jenna wins. For how many possible rolls would both girls win?
- 1
- 2
- 3
- +
- 4
- 5
- 6
- -
- 7
- 8
- 9
- $\frac{a}{b}$
- .
- 0
- =
- %
- $a^n$
- $a^{\circ}$
- $a_n$
- $\sqrt{}$
- $\sqrt[n]{}$
- $\pi$
- $\ln{}$
- $\log$
- $\theta$
- $\sin{}$
- $\cos{}$
- $\tan{}$
- $($
- $)$
- $[$
- $]$
- $\cap$
- $\cup$
- $,$
- $\infty$
Solution
We are trying to determine how many possible combinations of two numbers between 1 and 20 would make it so that the first number is a factor of the second number and the second number is a factor of the first number. We know that all of the positive factors of a positive number, except for the number itself, are less than the number. Therefore, if Jenna's number is greater than Mary's number, Jenna's number cannot be a divisor of Mary's number, and Jenna cannot win.
Likewise, if Jenna's number is less than Mary's number, Mary's number must be greater than Jenna's number, Mary's number cannot be a divisor of Jenna's number, and Mary cannot win. If Jenna's number is equal to Mary's number, both girls' numbers are factors of the other's number because any number is a factor of itself. Thus, we can determine that for both girls to win, they must roll the same number. Since there are 20 numbers on the dice, this means that there are $ \boxed{20} $ rolls for which both girls would win.
Freeflows