Question 1162818
<pre>

I'll change the numbers so you'll have to do something other than type them
in and get credit for what somebody else did.
</pre>You need 605 mL of a 75% alcohol solution. On hand, you have a 45%
alcohol mixture. How much of the 45% alcohol mixture and pure alcohol will
you need to obtain the desired solution?<pre>
        |How many ml?|Strength| ml of pure alcohol
-------------------------------------------------
Solution|     x      | 0.45   |     0.45x
Alcohol |     y      | 1.00   |     1.00y
---------------------------------------------
            605        0.75         0.75(605) 

The equations come from the first and last columns:

             x + y = 605
     0.45x + 1.00y = 0.75(605)
    --------------------------

Multiply the second equation through by 100,
that is, move the decimal 2 places right on one factor:

             x + y = 605
        45x + 100y = 75(605)
    --------------------------

Multiply 75 times 605 get 45375

             x + y = 605
        45x + 100y = 45275
    --------------------------

Solve the first equation for x

                 x = 605 - y

Substitute that for x in the second equation

        45x + 100y = 45275
45(605 - y) + 100y = 45275
27225 - 45y + 100y = 45275
       27226 + 55y = 45275
               55y = 18150
                 y = 18150/55
                 y = 330 ml of pure alcohol

Substitute in

                 x = 605 - y
                 x = 605 - 330
                 x = 275 ml of mixture

Now do the same with your numbers.

Edwin</pre>