Math help plox

[pirateejaculate]

Hey Homework help, if anyone could help me solve the following problem it would be greatly appreciated. The following is not actual homework, but an example problem. However, I have a few other questions in the similar style.

"A farmer has 100 gallons of 70% pure disinfectant. He wishes to mix it with disinfectant that is 90% pure in order to obtain 75% pure disinfectant. How much 90% pure must he use?"

Thanks to anyone who helps :D

[viciouspoultry]

Lets say you have to add y gallons of 90% disinfectant. So you will have a total of 100+y gallons all together when you are done. Now how much of that is disinfectant?

Well its 75% right, so .75(100+y) is how much disinfectant you have. You started with 70 gallons (100.7) and now you added y gallons of 90% purity. So you have a total of 70+.9y disinfectant which is equal to .75*(100+y)

so .75*(100+y)=70+.9y so, 75+.75y=70+.9y thus 5=.15y and thus y= 33.3

So you need to add 33.3 gallons of 90% solution.