#### Explanation:

$P \left(\text{doubles}\right) = \frac{1}{6}$
$\textcolor{w h i t e}{\text{XXX}}$One way to see this is to recognize that whatever
$\textcolor{w h i t e}{\text{XXX}}$value shows up on the face of the first die
$\textcolor{w h i t e}{\text{XXX}}$there is a $\frac{1}{6}$ that that value will show up on the second die.

So on average a player can expect to win 1 roll out of each 6 attempts.

For a fair game
color(white)("XXX")"amount to be won" xx"probability of winning" = "cost of each play"

In this case
color(white)("XXX")$90 xx 1/6 =$15