To find out how much money Mike spent on stamps, we can calculate the total cost of each type of stamp separately and then add those amounts together.
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Calculate the cost of the 20 stamps that cost $0.45 each: \[ 20 \text{ stamps} \times 0.45 \text{ dollars/stamp} = 9.00 \text{ dollars} \]
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Calculate the cost of the 15 stamps that cost $0.32 each: \[ 15 \text{ stamps} \times 0.32 \text{ dollars/stamp} = 4.80 \text{ dollars} \]
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Now add the two amounts together: \[ 9.00 \text{ dollars} + 4.80 \text{ dollars} = 13.80 \text{ dollars} \]
Therefore, Mike spent a total of $13.80.