Question

How many positive 3-digit integers contain only odd digits?

Answer 1

first digit has to be 1,3,5,7,9
as does second, and third.

number odd numbers= 5*5*5=125
and curiously, the number of even digits must be 4*5*5 (lead digit cannot be zero)=100
Now, containing one odd digit.
4*5*5+5*5*5+4*5*5=325 in two instances, first digit is only of four choices..
containg two odd digits
2*5*5*5 + 4*5*5 (second term is first digit is even)= 250+100

all of those should add to 9*10*10=900
checking: 350+325+100+125=900

Answer 2

125