Question 401189
In words:
(quarts of antifreeze started with (-) quarts of antifreeze drained off (+) quarts of antifreeze added) / ( final quarts of solution) = 50%
Let {{{x}}} = quarts of solution drained off
Then {{{x}}} = quarts of 100% antifreeze added
given:
{{{16}}} = final quarts of solution
{{{.4*16 = 6.4}}}  = quarts of antifreeze to start with
{{{.4x}}} = quarts of antifreeze drained off
------------------------------------------
{{{(6.4 -.4x + x) / 16 = .5}}}  (note: {{{16}}} is really {{{16 - x + x}}})
{{{6.4 - .4x + x = 16*.5}}}
{{{.6x = 8 - 6.4}}}
{{{.6x = 1.6}}}
{{{x = 2.667}}} quarts
2.667 quarts of 40% solution must be removed and 2.667 quarts of 
antifreeze must be added
check answer:
{{{(6.4 -.4*(8/3) + 8/3) / 16 = .5}}} 
{{{6.4 - 3.2/3  + 8/3 = 8}}}
{{{3*6.4 - 3.2 + 8 = 24}}}
{{{19.2 - 3.2 + 8 = 24}}}
{{{16 = 16}}}
OK