A 10% salt solution and a 40% salt solution are used to make 10 liters of a 30% salt solution. How many liter of the 10% salt solution are u

There is a formula that I use for problems like this:

(Percent)·(Amount) + (Percent)·(Amount)  =  (Final Percent)·(Final Amount)

If you have 10 liters divided into two parts, call the amount of one part 'x' and the amount of the other part '10 - x':

(10%)·(x) + (40%)·(10 - x)  =  (30%)·(10)

(0.10)·(x) + (0.40)·(10 - x)  =  (0.30)·(10)

0.10x - 4.00 - 0.40x  =  3.00

-0.30x - 4.00  =  3.00

-0.30x  =  -1.00

x  =  -1.00 / -0.30  =  3.33 liters

Just as GL said!

Same as the last one, crunch numbers until you get a ratio. In this case, it will be two parts 40% solution and one part 10% solution.

$${\frac{{\mathtt{1}}}{{\mathtt{3}}}}{\mathtt{\,\times\,}}\left({\mathtt{10}}\right) = {\frac{{\mathtt{10}}}{{\mathtt{3}}}} = {\mathtt{3.333\: \!333\: \!333\: \!333\: \!333\: \!3}}$$

So 3.333... liters of 10% solution were used.