There are 20 cows and you are required killl all of them in 7 days and all days should be utilized.?

It is not possible as the previous person stated. three odd numbers will never equal a even number.
1+1+1+1+1+1+1=7 so even if you go and add 2 any where or any even numbers it wont change that the ending result number will have to equal a odd number and not a even one.