Permutation&Combination

Probability Level pending

How many intergers are there betwen 100 and 1000 that contains only one 7?

1 solution

Abdullah Ahmed
Sep 21, 2016

first we consider the number $_$ $_$ $7$

without 7 we can choose the first gap is 8 and second gap is 9

the possible value is = $8*9$ = $72$

now we consider the number $_$ $7$ $_$

without 7 we can choose the first gap is 8 and third gap is 9

the possible value is = $8*9$ = $72$

last we consider the number $7$ $_$ $_$

without 7 we can choose the second gap is 9 and third gap is 9

the possible value is = $9*9$ = $81$

so $2*72+81=225$