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A certain paint colour is created by mixing 3 parts of red with 5 parts of blue.
How many gallons of red paints are needed to mix 40 gallons of this colour
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Pay attention on how I edited your post in order for it would make sense.
In all, there are 3 + 5 = 8 equal parts.
Each part is 40/8 = 5 gallons.
Hence, the amount of red paint is 3*5 = 15 gallons. ANSWER
Solved.