A bag contains 8 red balls and x blue balls, the odd against drawing a blue ball are 2: 5. What is the value of x?                                                               (Ans:20)

Ans:         No. of blue balls be x

No. of red balls be 8

Total no. of balls = x + 8

Probability of drawing blue balls = x/8 + x

Probability of drawing red balls =  8/8 + x

8/8 + x :     x/8 + x  = 2 : 5

2 (x/8 + x) = 5 (    8/8 + x    )

2x = 40

∴ x = 20