There are some people in a cinema. 3/5 of the people in the cinema are children. For the children in the cinema, number of girls:number of boys = 2:7. There are 170 girls in the cinema. Work out the number of adults in the cinema.

We can begin by working out the total number of children in the cinema. We know that for every 2 girls, there are 7 boys, which is the same as saying for every 1 girl, there are 3.5 boys. Three and a half boys sounds a bit silly, but stick with me! :)So, as there are 170 girls, there are 170 x 3.5 = 595 boys.Adding the girls and the boys together, we get 170+595=765 children in total.
Now, we know already that these children make up 3/5 of the total number of people, which means adults must make up 2/5 of the total.If 765 children is equal to 3/5, then we work out 1/5 by dividing this by 3, and we get 765/3=255.But the adults make up 2/5, so we have to multiply this number by two! Therefore there are 2 x 255 = 510 adults.