having trouble arranging this problem: 39c for each ounce plus 24 c for each additional ounce if spent $15 for a total of 45 stamps of these two denominations. how many stamps of each. thanks

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Let's say we have x number of 39 cent stamps.  If that is the case, and we have 45 total stamps, then we must have 45 - x for the number of 24 cent stamps.  For totals, we always multiply cost times amount.  So, for each type of stamp we have these expressions for their totals:39cent: 39 * x = 39x24cent: 24 * (45 - x) = 24(45 - x)Now, we know from the problem that these add up to a total of $15.  For our equation, though, we will need to also use cents for our unit.  There are 100 cents in every dollar, so we have 1500 cents for the total.  We just make an equation now where our two expressions add up to 1500.39x + 24(45 - x) = 1500Now, we solve for x, first distribute the 2439x + 1080 - 24x = 1500Combine like terms on the left side15x + 1080 = 1500Subtract 1080 from both sides15x = 420Divide both sides by 15x = 28We have 28 of the 39cent stamps.  To get how many 24cent stamps we have, just subtract 28 from 45.45 - 28 = 17