The question: A postal cler sold 75 stamps for $19.95. Some were 20-cent and some were 33-cent stamps. How many of each kind did he sell?

What I did so far:

but 13 doesn't go into 495 evenly. What did I do wrong?

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  1. You need to set up a system of equations, which means we need two different equations. (You were on the right track by modeling the situation with equations, though.)

    Let a = the number of 20-cent stamps.
    Let b = the number of 33-cent stamps.

    a + b = 75
    .20a + .33b = 19.95

    We can use one of two methods to solve this system: substitution or elimination. Let's use substitution for this.

    From a + b = 75, we know that a = 75 - b. Plug that into our other equation.

    .20a + .33b = 19.95
    .20(75 - b) + .33b = 19.95
    15 - .20b + .33b = 19.95
    15 + .13b = 19.95
    .13b = 4.95
    b = 4.95/.13 = 38.07

    That's the same issue you had... let's just round b to 38, and see what we come up with for a.

    a + b = 75
    a + 38 = 75
    a = 37

    Now let's check with the other equation.

    .20a + .33b = 19.95
    .20(37) + .33(38) = 19.95
    7.40 + 12.54 = 19.95
    19.94 = 19.95
    (Actually, it doesn't... but I guess we just have to say it does.)

    He sold 38 33-cent stamps and 37 20-cent stamps.

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  2. The question: A postal cler sold 75 stamps for $19.95. Some were 20-cent and some were 33-cent stamps. How many of each kind did he sell?

    Lets explore this ignoring the fact that there are supposed to be 75 stamps.

    1--.20a + .33b = 19.95 or 20a + 33b = 1995
    2--Dividing thrugh by the lowest coefficient yields 1a + 1b + 13b/20 = 99 + 15/20
    3--(13b - 15)/20 must be an integer.
    4--We want the coefficient of b to be 1
    5--Multiplying the numerator by 13 yields (221b - 255)/20
    6--Dividing by 20 again yields 11b + b/20 - 12 - 15/20
    7--Now, (b - 15)/20 = k making b = 20k + 15
    8--Substituting back into (1) yields a = 77 - 33k
    10--Conclusion: No mixture of 75 20 cent and 33 cent stamps can total $19.95 evenly. 77 stamps is the closet total that will sum to $19.95 evenly.

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