Sarah went to an electronics store, purchased 3 cds and 5 dvds, and spent $137. michelle went to the same store, bought 2 cds and 7 dvds, and spent $161. how much does each cd sell for at the store?

You get the following two equations:3x + 5y = 1372x + 7y = 161
If you have two equations, you can solve for two variables.
Solve for x in the first equation:
x = (137-5y)/3
Then substitute this in for "x" in the 2nd equation.
2[ (137-5y) / 3 ] + 7y = 161
Now you can solve for "y"
(274/3) - (10/3)*y + 7y = 161
(11/3)*y = 161 - (274/3)
y = 19
Now that you have "y", you can solve for "x"
2x + 7(19) = 161
x = 14
CDs cost $14 and DVDs cost $19 each

The answer is 30 grams. have fun