Question 118882
You have q quarters, d dimes, and p pennies.  Each quarter is worth 25 cents, so the value of your quarters is 25q.  Likewise the value of your dimes is 10d and the value of your pennies is p.


We know that:

1) {{{q+d+p=15}}}


2) {{{25q+10d+p=129}}}  ($1.29 is 129 cents)


3) {{{q=2d}}}


The first thing we can do is substitute 2d for q in each of equations 1 and 2.


4) {{{2d+d+p=15}}} => {{{3d+p=15}}} and


5) {{{50d+10d+p=129}}} => {{{60d+p=129}}}


Now, solve equation 4) for p


6) {{{p=15-3d}}}


And substitute this value for p into equation 5)


{{{60d+(15-3d)=129}}}


Combine terms and solve for d


{{{57d=129-15=114}}}


{{{d=114/57=2}}}


Now we know we have 2 dimes.  That means we have {{{2*2=4}}} quarters.  2 dimes plus 4 quarters is 6 coins.  15 coins minus 6 coins is 9 coins, so we have 9 pennies.


Check:

{{{4 + 2 + 9= 15}}}


{{{25*4+10*2+9=100+20+9=129}}} Answer checks


Hope you aren't hungry for a Big Mac.