A gas station sells three types of gas Regular for 295 a gal

A gas station sells three types of gas: Regular for $2.95 a gallon, Performance Plus for $3.10 a gallon, and Premium for $3.25 a gallon. On a particular day 4400 gallons of gas were sold for a total of $13,445. Two times as many gallons of Regular as Premium gas were sold. How many gallons of each type of gas were sold that day?

Solution

r + pp + p = 4400 gallons
2.95r + 3.10pp + 3.25p = 13445
r = 2p
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Substitute::
3p + pp = 4400
9.15p + 3.10pp = 13445
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Modify::
9.3p + 3.10pp = 13640
9.15p + 3.10pp = 13445
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Subtract:
0.15p = 195
p = 1300 (# of gallons of Premium sold)
r = 2p = 2600 (# of gallons of regular sold)
pp = 4400 - 3900 = 500 (# of gallons of Premium Plus sold)