# Math question for attack roll

### Rules Discussion

Sorry math was never my strong suit, I'm trying to figure out the probability of rolling an 8 or more on a D20 with two chances at it. I know for one chance it's 65% but how would you figure it out for two rolls? Thanks all for the help.

 1 person marked this as a favorite.

1-(35% * 35%) = 0.8775

87.75%

 1 person marked this as a favorite.

You take the 35% chance of failing the first.

Then you calculate 65% of 35% which gives you 22.75%. This is the chance that you'll get less than 8 the first time but then get 8 or more the second.

Now you either add 65% + 22.75% or you subtract 100% by (35% - 22.75%). Both should give you 87.75% which is the total chance of you succeeding at either roll.

 1 person marked this as a favorite.
Paizo Superscriber; Pathfinder Companion, Pathfinder Accessories Subscriber; Starfinder Superscriber

You're asking for what's the probability that at least one roll out of two, is 8 or more? Its often simpler to express it as the failure case -- what's the chance of both rolls being 7 or lower.

Chance of 1 roll being 7 or lower: 35%
Chance of both rolls being 7 or lower : 0.35^2 = 12.25%
1.0 minus that: 0.8775 = 87.75%