50%
Explanation:
Tounge rolling is a dominate trait, so that means it would have to have at least 1 captial letter in it't genotype (assuming captial letters refer to the dominate trait). But, because the demonate trait covers any non-dominate traits, you can have a lower case letter in the genotype while still having someone be a tounge roller (assuming lower case letters are non-dominate traits)
Because non-tounge rollers are non-dominate, they must have two lower case letters in their genotype.
This leaves you with the two genotypes of :
AA or Aa (tounge rollers)
aa (non-tounge rollers)
When crossing AA with aa in a punette, every outcome is Aa. This leaves you with no potentional non-tounge rollers. But, you can still cross Aa with aa, because as said before Aa has a dominate gene, so it would result in someone with the tounge rolling ability.
When crossing Aa and aa in a punnet sqaure, you can get outcomes. Aa or aa. 2 of teh 4 options are Aa, and 2 of teh four options are aa. This means half, or 50% of the offspring are predicted to be non-tounge rollers, showing the high ercentage must be 50%.