A bag contains red, blue and black balls. The ratio of red to blue balls is 3:5. The ratio of black to red balls is 3:4. Options: [1] 5:3 [2] 4:5 [3] 9:20 [4] 20:9

 A bag contains red, blue and black balls. The ratio of red to blue balls is 3:5. The ratio of black to red balls is 3:4.

Options: 
[1] 5:3     [2] 4:5     [3] 9:20      [4] 20:9 

Answer: 
[4] 20:9 

Given sequence of balls is red, blue, and black.
red : blue = 3 : 5        and        black : red = 3:4 
Since the red ball is common in both ratio
so, 
I try to make the quantity of red ball equal in both ratio,
i,e    red : blue = 12 : 20        and        black : red = 9 : 12
here it can be find that the ratio  of blue to black is 20 :9