Question 798597
Vol 40%: n
35 @ 15% = 525
n  @ 40% = 40n
Total 35+n  @ 30% = 525+40n
So {{{ (525+40n)/(35+n)=30 }}}
525 + 40n = 30*35 + 30n
Subtracting (30n + 525) gives
10n =  1050-525 = 525
n = 52.5ml
Check :
{{{ (525+52.5*40)/(35+52.5) = (525+2100)/87.5}}}
={{{2625/87.5=30%}}}