I added both days together and divided by 2 because there are two individuals working on 20 trees and divided equally it would be 10 and it would take 5 days because that’s half.
As you can notice, in the parenthesis, we have a multiplication between the variable x and the number 2. Since we can't multiply a variable with a number, x.2 becomes 2x. So the reason for statement 2 is multiplication.