How do i find the missing side length?

Can someone me out here number 4 plz

Asurvey of 2,000 doctors showed that an average of 3 out of 5 doctors use brand x aspirin.how many doctors use brand x aspirin

White shapes and black shapes are used in a game. some of the shapes are circles. while the rest are squares. the ratio of the number of white shapes to the number of black shapes is 5: 11. the ratio of the number of white cicrles to the number of white squares is 3: 7. the ratio of the number of black circles to the number of black squres is 3: 8. work out what fraction of all the shapes are circles.