Question 206045
I get x + 1 as an answer for your first question.
Pick a a number:x
Multiply that number by 5: 5x
Add ten to the product: 5x + 10
Divide the sum by 5: (5x + 10) / 5 = (5x/5)+(10/5)= x + 2
Subtract 1 from the quotient: x + 2 -1 = x + 1

As for the other questions, I don't understand what you mean by 'estimate'.
Those problems could be solved directly by addition and multiplication.