hi jacks,

You may throw a one straight away, or on throw 2, or on throw 3, or on throw 4, or ...........

See diagram.

P(one at first throw) = 1/6

P(one at second throw) = 5/6 times 1/6 = 5/36

P(one at third throw) = 5/6 x 5/6 x 1/6 = 25/216

etc etc.

As this goes on for ever you need to know how to do the sum to infinity of a geometric progression. Post again if you want help with that.

Bob