Number System
A bag contains four red, six black and seven green balls. Three balls are drawn randomly. What is the probability that the balls drawn contain exactly two red balls ?
Total balls = 4 + 6 + 7 = 17∴ n(S) = 17C1 = 680Two red balls can be selected from four red balls in 4C2 = 6 ways.and the third ball can be selected from the remaining 13 balls in 13C1 = 13 ways.∴ P (E) = 13x6/680 = 39/340
Divisibility of 11: If the difference of the sum of the digits at odd places and the sum of the digits at even places of a number is zero or a multiple of 11, then that number is divisible by 11.As per divisibility rule of 11, difference = (x + 21) - 18 = x + 3=> x + 3 = 11 => x = 8
EXAMIANS