The weights of 4 boxes are 20, 40, 50 and 30 kilograms. Which of the following cannot be the total weight, in kilograms, of any combination of these boxes and in a combination a box can be used only once?
In the following problem, '=' stands for '÷' , '+' stands for '' , 'x' stands for '=', '' stands for '+' and '÷' stands for 'x'. Find the correct equation.