Question 147686
-----With two variables:   # of 10 cent stamps is x. # of 15 cent stamps is y.  You have the prices of the stamps and the sum of the price of 60 stamps.  So first set up an equation {{{x+y=60}}}. Then set up {{{10x+15y=725}}}.  Then in the first equation solve for x.  You should get {{{x=60-y}}}. Plug that in the second equation and get {{{10(60-y)+15y=725}}}. Distribute the ten and get {{{600-10y+15y=725}}}.  Combine like terms {{{600+5y=725}}}.  Subtract 600 from both sides and get {{{5y=125}}}.  Then divide by five and {{{y=25}}}.  Then plug in y in the original equation and get {{{x=35}}}.  So she bought 35 10cent stamps and 25 15 cent stamps.



-----With one variable:   Also, you could start out with {{{10x+15(60-x)=725}}} since you know the sum to be 60.  Then distribute and get {{{10x+900-15x=725}}}. Combine like terms and get {{{-5x+900=725}}}.  Subtract 900 from each side and get {{{-5x=175}}}.  Divide by -5 and get x=-35.  Now, you can't buy a negative amount of stamps, so make 35 positive. {{{x=35}}}.  Then plug x into the original equation and get {{{y=25}}}.  So she bought 35 10cent stamps and 25 15 cent stamps.