Question

Biggest 3,5,7,8 digit even number
Smallest 3,5,7,8 digit even number

Answer 1

A number is even if it ends with an even digit, so 8 must be the last one.

Now, the digits to the left are more worth than those on the right, so if you want a big number you have to stack the greatest digits to the left, otherwise if you want a small number you have to stack the smallest digits to the left.

So, the greatest number is 7538, the smaller is 3578.