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Message: <blockquote data-quote="ECMO3" data-source="post: 8499468" data-attributes="member: 7030563">Bob gets ready to cast "instant win" followed by SB.  75% of the time SB will have no effect at all on the outcome of "instant win".  He will have set all this up for nothing.50% of the time the exact same thing would have happened if Bob did not prepare SB at all.  He could have prepared any other spell instead of SB and the outcome of this encounter would be the same.25% of the time the same thing would have happened without also using a 1st level slot. If Bob has the opportunity to cast SB his 1st level slot will cause the monster to have a 50% of losing when he casts it, not 75%.  He spends this knowing there is a 50% chance the monster will succeed."Instant Win" sounds powerful, so I assume it is a 6th or higher level spell.  If instant win is a 6th or higher level spell, Bob will get traction out of using SB with instant win once every 4 days on average (assuming he casts it every day in position and saves a slot) if he is lucky enough to encounter monster or someone else who needs an 11 to save (it is actually less if the save is higher or lower than 11).  The other 3 days SB will be a complete waste unless he uses it for something other than pairing with instant win.Your total chance of landing instant win when you have SB ready is 50% not 5/8.  SB can only affect 1 dice not both, so you take the highest dice (assuming it succeeds) and compare that with the reroll taking the lowest.  Then you go back and take the higher of this new value or the lower initial roll. (PHB page 173).The result is, Instant win is successful when the the monster rolled lower than the DC on both of his rolls (25%) or when he rolled lower than the DC on only one of his two rolls and also rolls lower than the DC on the SB reroll.  If monster rolls above the DC on both his rolls with advantage Bob can not change the outcome regardless of the SB reroll.  You have a 25% chance of not needing SB, a 50% chance of casting it and failing and a 25% chance of casting it and succeeding.Here is how to do it mathematically:If monster gained advantage there are 3 dice in play and there is a methodology to applying this.  Dice A, Dice B, Dice S.First you take the higher of A or B - max(A,B)If max(A,B) succeeds on the save you spend the spell slot and take the lower of that dice and S  - min(S,max(A,B))Then you take the higher of this dice and the dice left behind on the first roll max(min(A,B),min(S, max(A,B))) to account for advantage.  This is what is perscribed in the PHB when you replace a dice on a roll with advantage or disadvantage.  In  boolean notation it would be (S&(A|B))|(A&B). It is a lot easier to do when you are looking at the dice than trying to think through it.Here is per die  outcome (1 is success, 0 is failure)A-B--S0-0--X  SB not cast, Result Fail  25%0-1--0  SB replaces 1, Result Fail 12.5%1-0--0  SB replaces 1, Result Fail 12.5%1-1--0 SB replaces 1, Result Succeed 12.5% - This is the one you were counting as a failure in 5/8.0-1--1 Result Succeed 12.5%1-0--1 Result Succeed 12.5%1-1--1 Result Succeed 12.5%Again though, SB has no effect on the outcome 75% of  the time. It only affects the outcome when the conditions in red above happen. This is using it with a DC 11 (50%)  if you change that DC in either way, harder or easier, this number will go down.Further when you cast SB in this case it only has a 33% chance of working and changing the result to a fail.  When the DM says "monster saves, do you want to cast SB" - you know monster has a 67% chance of succeeding in his save if you cast it.  If you know the DC is 11 and monster has advantage when you cast SB you know it is not going to work most of the time when you use your reaction and spend your slot.Also the overall chance of "Instant Win" working in this case is the same as being able to cancel advantage only because  the DC is 11.  If you make the DC easier canceling advantage will be more effective numerically, if you make it harder SB has less of a chance of affecting the outcome but a higher overall chance of landing "instant win".</blockquote>

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