Math Is Fun Forum

fusilli_jerry89

Member

Registered: 2006-06-23

Posts: 86

Questions I forgot how to do

1) Factor:

x^4-81y^8
wait this one must be (x^2+9y^4)(x^2-9y^4)

2) You have two large containers. The first contains 9% solution and the second 12%. To make 4 litres of 10% solution, how many litres of each solution should you use?

I did this and ended up getting 12.67 and -8.67. How can it be negative?

Last edited by fusilli_jerry89 (2006-10-12 12:27:36)

pi man

Member

Registered: 2006-07-06

Posts: 251

Re: Questions I forgot how to do

I must have made the same mistake that you did because I got the same answer the first time.   Think about this.   What if you used 1 liter of 9% and 3 liters of 12%?   You add 9 + 12 + 12 + 12 AND  then divide by 4.   I forget the "divide by 4" as you probably did.   We're essentially calculating the average concentration. 

Let X be the amount of 9% solution.  4 - X is the amount of 12% solution.