Question

How many three digit numbers can you get by arranging the numbers 5, 6,

7, 8, 3, 4, 5, 6, 7, 8, 9?

Answer 1

84 numbers

Explanation:

Numbers starting with 12 – 7 numbers

Numbers starting with 13 – 6 numbers;

14 – 5, 15 – 4, 16 – 3, 17 – 2, 18 – 1.

Thus total number of

numbers starting from 1 is given by the sum of 1 to 7 = 28.

Number of numbers starting from 2- would be given by the sum of 1 to 6 = 21

Number of numbers starting from 3- sum of 1 to 5 = 15

Number of numbers starting from 4 – sum of 1 to 4 = 10

Number of numbers starting from 5 – sum of 1 to 3 = 6

Number of numbers starting from 6 = 1 + 2 = 3

Number of numbers starting from 7 = 1

Thus a total of: 28 + 21 + 15 + 10 + 6 + 3 + 1 = 84 such numbers.