21 In a group of some people, 15 don't take any of three vitamins; A, B and D. The number of people who take only one vitamin is 15 more than the number of people who take only two vitamins and the number of people who take only two vitamins is 20 more than that of the people who take all the three vitamins. If the number of people who take only one vitamin is equal to the number of people who take at least two vitamins, how many people are there in the group? ​