Question 897265
It is really
20 pennies + 25 other coins (nickels and dimes) = 220 cents = $2.20 .
{{{N}}}= number of nickels
{{{D}}}= number of dimes
You know that the combined total of nickels and dimes is 23 coins, so
{{{N+D=25}}} .
That is one equation.
You know that the value (in cents) ofthe whole amount of coins in the piggy banks is
{{{20+5N+10D=220}}} <--> {{{5N+10D=220-20}}} <--> {{{5N+10D=200}}}
and dividing both sides of the equal sign by {{{5}}} you get
{{{5N+10D=200}}} <--> {{{(5N+10D)/5=200/5}}} <--> {{{N+2D=40}}}
That is a second equation.
you have a system of equations: {{{system(N+D=25,N+2D=40)}}}
There many ways to solve it, but subtracting the first equation from the second, we get {{{D=40-25=highlight(15)}}} , and
{{{system (D=15,N+D=25)}}} ---> {{{highlight(N=10)}}}
 
IF you do not know what a system of equations is (or how to solve it),
you would start with {{{N+D=25}}} ---> {{{N=25-D}}} .
Then {{{25-D)= number of nickels
Then you would write the expression for the total value as
{{{20+5(25-D)+10D=220}}}
{{{20+125-5D+10D=220}}}
{{{20+125+5D=220}}}
{{{145+5D=220}}}
{{{5D=220-145}}}
{{{5D=75}}}
{{{D=75/5}}}
{{{highlight(D=15)}}} .
That is the number of dimes, and you know that the number of nickels is
{{{25-D=25-15=highlight(10)}}} .