Question

How many litres of a 90% solution of concentrated acid needs to be mixed with a 75% solution ofconcentrated acid to get a 30 l solution of 78% concentrated acid?

Answer 1

concentration is given which is with respect to 100, hence we can take x litre of 90% and (30-x)of 75% 
x*90 + (30-x)*75 = 30*78
90x -75x+2250 = 2340 
15x = 90 
x = 90/15 = 6 litres

Answer 2

"The concentration of solution is given with respect to 100.

Now, we take x litre of the 90% solution as well as (30-x) of 75%.

The explanation is given below.  

30*78 = x*90 + (30-x)*75

2340 = 90x -75x+2250

90 = 15x

90/15=x

6 = x.

Finally, we find the value of x is 6, so 6 litres of solutions.

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