Question
In a park, the ratio of oak to pine trees is 7:4. If 21
oak trees and 12 pine trees are planted, the ratio of pine trees to oak trees becomes 3:5. Find the original number of pine trees.Solution
ATQ, Let the initial number of oak trees be 7x and pine trees 4x. After adding, oak trees = 7x+21, pine trees = 4x+12. New ratio (pine to oak): (4x+12/7x+21) = 3/5 Cross-multiplying = 
Original number of pine trees = 4x = 4×3 = 12.
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