How many three digit numbers can be formed without using the digits 0,2,3,4,5 and 6?

Explanation

Clearly, we have to form 3-digit numbers by using the digits 1, 7, 8 and 9 while the repetition of digits is allowed.
Clearly, each one of the unit's ten's and hundred's places can be filled by any of the digits 1, 7, 8, 9, i.e., in four ways.
Hence, the numbers of required numbers `=(4xx4xx4)=64.`