Question

Let A be a four digit number and let B be a three digit number so that A-b Is a single digit number. How many possible pairs (a,b) are there?​

Answer 1

45

Explanation:

1000-999 through 1000-991 (9 pairs)

1001-999 through 1001-992 (8 pairs)

1002-999 through 1002-993 (7 pairs)

1003-999 through 1003-994 (6 pairs)

1004-999 through 1004-995 (5 pairs)

1005-999 through 1005-996 (4 pairs)

1006-999 through 1006-997 (3 pairs)

1007-999 through 1007-998 (2 pairs)

1008-999 (1 pair)