Even number between 70 and 85. the sum of the digits is an even number less
than 10. what is the number?

To be between 70 and 85 they have to have a first digit of 7 or 8. The only
even numbers less than 10 for the two digits to add up to are 2,4,6, and 8.
Since the first digit is 7 or 8, the sum of the digits can't be 2,4, or 6, so
that leaves only 8.  The only two possibilities then are 71 and 80 and 80 is
the only even one.  

Edwin