Algebra
posted by John's
A car's cooling system of a car has a capacity of 15 liters. suppose that it is currently full of a 40% antifreeze mixture. you want 60% antifreeze mixture so you drain some of the current mixture and then add the pure antifreeze. How much antifreeze should be added?

Reiny
you must drain the same amount as the amount you are going to add , to keep 15 L in there
let that amount be x L
so we are draining .4x L of antifreeze and then adding x L of pure antifreeze
so .4(15)  .4x + x = .6(15)
times 10
4(15)  4x + 10x = 6(15)
60 + 6x = 90
6x = 30
x = 5 
Anonymous
i don't have an answer but i have a question:
At the end of Summer, Puss discovers that his radiator antifreeze solution has dropped below the safe level. If the radiator contains 4 gallons of a 25% antifreeze solution, how many gallons of pure antifreeze must he add to bring it up to a desired 50% solution?
i am not sure how to solve this can you please put a solution to this problem
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