Each member of a class sampled a piece of ptc testing paper. thirty people sampled the chemical. twenty persons were tasters. how many had heterozygous genes?

The correct answer for this question is this one: 

Given that there are 30 people, then there are 60 alleles. 10 of these people are not tasters. So, there are 20 of those 60 are t alleles. So, 40 alleles are left, but of a mix of t and T. It said in the lesson text that people who can taste this have at least one dominant gene "T". So it could either be homozygous (T,T) or heterozygous (T,t or t,T). People with homozygous recessive genes (t,t) can't taste it.