Probability of an event =
(number of ways the event can happen) / (total number of possible outcomes) .
Well, the total number of possible outcomes is easy. It's
(6 possibilities for the first cube) x (6 possibilities for the 2nd cube)
= 36 total possible ways for two cubes to roll.
Now, how many of those possibilities show either 4 or 7 ?
Well, here's how a 4 can come up:
1 . . . 3
2 . . . 2
3 . . . 1
and here's how a 7 can come up:
1 . . . 6
2 . . . 5
3 . . . 4
4 . . . 3
5 . . . 2
6 . . . 1 .
So there are 9 ways all together for Lenny to be successful.
The probability is (9/36) .
That's 1/4 , or 25% .