Rudolff's Problem

Rudolff's book, called Coss, written in 1525, was the first German algebra book. The reason for the title is that cosa is a 'thing', a term used for the unknown. Algebraists were called cossists, and algebra the cossic art, for many years. Here is the problem:
A group of 20 people pay a total of £ 20 to see an exhibition. The admission price is £ 3 for men, £ 2 for women and 50p for children. How many men, women and children are there in the group?