The weights of 4 boxes are 30, 40, 50 and 80 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?

If L=+, M= - , N = ×, P=÷ then 5 N 5 P 5 L 5 M 5 =?

Adam who is 20 years old is 4 times as old as Mary. What will be Mary’s age when Adam is twice as old as her?

The average age of three friends A, B, C is 20 years. The average age of A and B is 19 years, then what is the age of C?

In an office of 50 people, 18 write with their right hand, 26 write with their left hand and 2 can write with both hands. How many cannot write? 

The sum of the present ages of two cousins is 46 years. Eight year ago, the elder one was twice as old a the younger one. What is the present age of the elder cousin?

The following equation is incorrect. Which two signs should be interchanged to correct the equation?
12 + 10 - 28 ÷ 7 x 4 = 48