A number of girls bought red, green and black ball pens. Three bought one of each colour. Of the girls who bought two colours, 3 did not buy red,5 not green and 2 not black. The same number of girls bought red only as bought red with other colours. The number bought black only as bought green only. More girls bought red and black but not green than bought black only but more girls bought green only than bought green and black but not red. How many girls were there?