Thank you!

For b) average of 5 numbers is multiple of 5, answer is 56

Total of 4 number = 644
Average = 161

The next average is multiple of 5 = 165

165×5 = 825
825-644= 181(not a 2 digit even number)
So, the average should be less than 161 since the new number is 2 digit even number.
So average either
160
155
150
145
140...

644+56=700
Average = 140 (multiple of 5)