The number of 3-digit numbers, formed using the digits 2, 3, 4, 5 and 7, when the repetition of digits is not allowed, and which are not divisible by 3 , is equal to .
Answer
36
Solution
A number is divisible by 3 only when its digit sum is divisible by 3, so we pick the digit sets whose sum is not a multiple of 3.
Such sets from \2, 3, 4, 5, 7\ (digit sum): \2, 3, 5\ (10), \2, 4, 5\ (11), \2, 4, 7\ (13), \2, 5, 7\ (14), \3, 4, 7\ (14), \4, 5, 7\ (16), i.e. 6 sets
(the remaining sets \2, 3, 4\, \2, 3, 7\, \3, 4, 5\, \3, 5, 7\ have sums 9, 12, 12, 15, so they are left out)
Since digits are not repeated, each set gives 3! = 6 numbers.
Required number = 6 6 = 36
Answer: 36