Question

How many 2-digit numbers contain the digit 7 only once

Answer 1

18

Explanation:

Counting:

07 17 27 37 47 57 67 70 71 72 73 74 75 76 78 79 87 97

notice we can ALSO use variation with no repetition equation with n=9 (numbers from 0 to 10) and p=1 (two digits) but the value can be in the first position or the second position


`(9!)/((9-8)!)` +
`(9!)/((9-8)!)`
`=9+9=18`