13. How many pairs of two-digit positive integers have a difference of 50 ?
A 10
B 20
C 25
D 35
E 40
Solution
E

The smallest pair of two-digit integers with a difference of 50 is 10 and 60 . The largest pair of two-digit integers with a difference of 50 is 49 and 99 . Hence there are 40 pairs of two-digit integers with a difference of 50 .