_ _ _ _
since it has to be a four digit number and the restrictions are only two substitue the first digit with 1
1 _ _ _
then substitue the last one with 1 because there is only one multiple of 5 that is a single digit
1 _ _ 1
then substitue the second gap with 8 because 0-9 are ten numbers and since you already used 2 numbers 10-2 is 8
1 8 _ 1
then substitue the last gap with 7 because you already used three numbers
1 8 7 1
then multiply all of them and you get 56
