SOLUTION: Pedro is making brown paint by mixing 4 parts green paint with 5 parts red paint. He wants to make 36 gallons of brown paint. How many gallons of red paint will Pedro need?

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Let = gallons of red paint needed
Let = gallons of green paint needed
given:
(1)
(2)
----------------
From (2)
(2)
(2)
Multiply both sides of (1) by
and add (2) to (1)
(1)
(2)


and, since
(1)


16 gallons of red and 20 gallons of green are needed