How many four-digit numbers can be formed with the digits 3, 5, 7, 8, 9 which are greater than 7000, if repetition of digits is not allowed?
Given: Four-digit number is required which is greater than 7000
Assume four boxes, now in the first box can either be one of the three numbers 7, 8 or 9, so there are three possibilities which are 3C1
In the second box, the numbers can be any of the four digits left, so the possibility is 4C1
Similarly, for the third box, the numbers can be any of the three digits left, so the possibility is 3C1
for the fourth box, the numbers can be any of the two digits left, so the possibility is 2C1
Hence the total number of possible outcomes is 3C1 × 4C1 × 3C1 × 2C1 = 3 × 4 × 3 × 2 = 72