Question 1055108
c for capacity, c=15.
p for current percent antifreeze in the system, p=20.
T for the target percent wanted as result in the system, T=50.
v for volume of current 20% antifreeze to remove and replace with 100% antifreeze to reach T; v is unknown.


How much actual antifreeze in the system now?
{{{c*(p/100)}}}


How much actual antifreeze if remove v?
{{{c(p/100)-(p/100)v}}}


How much actual antifreeze when replace with v of 100%?
{{{c(p/100)-(p/100)v+v}}}


Total volume of result mixture will be kept the same, c.  The target concentration T is wanted.
{{{highlight((c(p/100)-(p/100)v+v)/c=T/100)}}}
Solve for v.


A lesson very much in this manner but maybe different variable names is here:
<a href="https://www.algebra.com/my/drain-and-replace-antifreeze.lesson?content_action=show_dev">the radiator antifreeze drain & replace problem</a>