Answer
Total number of four digit numbers less then 6000 can be formed using the digits 2, 4, 6, 7, 8, 9 = 144
Work Step by Step
Given four digit numbers less than 6000, then MSB (most significant bit) will be either 2 or 4. So number of ways to express MSB = 2
As given number is odd, then LSB (least significant bit) will be either 7 or 9. So number of ways to express LSB = 2
Middle both bits may be any of given number 2,4,6,7,8,9. So number of ways to express each of middle bit = 6
Total number of four digit numbers less then 6000c an be formed using the digits 2, 4, 6, 7, 8, 9 = 2$\times$6$\times$6$\times$2 = 144
