How many 4 digit numbers have the property that the last digit is at least 4 times the first digit?
If the first digit from the left is 1, then there are: 1 x 10 x 10 x 6 = 600 such numbers
If the first digit from the left is 2, yjen there are: 1 x 10 x 10 x 1 = 100 such numbers.
600 + 100 = 700 numbers in total
+128474 It's clear that the first digit must either be a 1 or a 2
If the first digit is a 1 , we have 6 possibilities for the last digit [4,5,6,7,8 or 9]
So there are 10 * 10 * 6 = 600 4-digit numbers of this type
If the first digit is a 2, we have 2 possibilities for the last digit [8 or 9]
So there are 10 * 10 * 2 = 200 4-digts of this type
So.....we have 600 + 200 = 800 4-digit integers
