Question 975494
{{{x}}}= number of 10-cent coins.
{{{y}}}= number of 20-cent coins.
{{{x+y=25}}}
{{{0.10x}}}= value of {{{x}}} 10-cent coins (in $).
{{{0.20y}}}= value of {{{y}}} 20-cent coins (in $).
{{{0.10x+0.20y}}}= value of {{{x}}} 10-cent coins plus {{{y}}} 20-cent coins (in $).
{{{0.10x+0.20y=3.50}}}<--->{{{x+2y=35}}} (multiplying both sides of the equal sign times 10)
 
You have two linear equations and two variables, forming a system of linear equations.
{{{system(x+y=25,x+2y=35)}}}
You could solve the system by substitution,
solving one equation for one of the variables,
for example {{{x+y=25}}}--->{{{x=25-y}}} ,
and then substituting the expression found for that variable into the other equation to find the value of the other variable:
{{{x+2y=35}}}--->{{{(25-y)+2y=35}}}--->{{{25+y=35}}}--->{{{y=235-25}}}--->{{{highlight(y=10)}}} ,
anfd finally, substituting the value found into the solved first equation to find the value of the other variable:
{{{x=25-y}}}--->{{{x=25-10}}}--->{{{highlight(x=15)}}} .