Fifty pounds of a cement-and-sand mixture is 40% sand. How many pounds of sand must be added for the resulting mixture to be 60% sand?
To get the answer you would have to multiply fifty by .4 to get 20. now you know that 20 is 2/5 of the bad and 1/5 must be 10. So you would multiply 10 by 3 to get to 3/5 so you can have 60%. The final answer is 30