Ok, this one isn't too hard.
State the amount of one kind of stamps X, in this case it's the 20 stamps, so that amount is called X.
For every stamp of 20 cent, we need two stamps of 32 cent, so for every X of 20, we need two X's of 32, so for every 20*X, we need 32*2X.
Work that one out and you get the solution
32(2x)+20x = 336.
64x + 20x = 336
84x = 336
x = 336/84
x = 4
So you need 4 20 cent stamps, and 8 32 cent stamps, if we recalculate that it perfectly fits
Edit: ninja'd, oh well.
Edit2: bored so I figured I'd do 2 as well.
Danielle spent $17 at an amusement park for admnission and rides. If she paid $5 for admission, and the rides cost $3 each, what is the total number of rides that she went on?
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Ok, so we gonna state the amount of rides, again, X. The total number of dollars she spent must be 17.
So that makes:
****** = 17.
She ALWAYS pays $5 entrance, no matter the amount of rides, we enter that in our formula:
***** + 5 = 17.
She pays 3 dollars for every ride, so every X she pays 3, that makes X*3
X*3 + 5 = 17
As I'm lazy 3x + 5 = 17
3x = 12
x = 12/3 = 4
So she did a total number of 4 rides.
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Mohammed keeps quarters, nickles and dimes in his change jar. He has a total of 52 coins. He has three more quarters than dimes, and five fewer nickles than dimes. How many dimes does Mohammed have?
Ok, again, outcome = 52
**** = 52
We state the amount of dimes = X, so the amount of quarters = x+3, amount of dimes = x, and amount of nickles = (x-5)
Makes
(X+3) + X + (X-5) = 52
X + 3 + X + X - 5 = 52
3X + 3 - 5 = 52
3X -2 = 52
3X = 54 Boreas: add 2 to both sides, you get 3X = 52+2
X = 54/3 = 18
First two are correct, I'm doubting about the 3rd one but it should be correct
Boreas: 3rd is correct too
Have fun!