This question is essentially just about ratios and the average formula. You can actually do the math in a few different ways, but sometimes it's easiest to just break simple calculations down into their components:
We're told that we have 14 liters of "forest green" paint. Forest green paint is 4 parts blue paint and 3 parts yellow paint. With this information, we know that the 14 liters we currently have is....
14 liters of forest green = 8 liters blue paint + 6 liters yellow paint
"Verdant green" paint is 3 parts blue paint and 4 parts yellow paint. We're asked how many liters of YELLOW paint must be ADDED to the 14 liters of forest green paint to convert that paint into verdant green paint.
Since Verdant green paint uses the ratio of blue to yellow as B/Y = 3/4, we can use the current amount of blue paint that we have to figure out how much TOTAL yellow paint we need.
With 8 liters of blue, we can solve for TOTAL yellow:
8/Y = 3/4
3Y = 32
Y = 10 2/3 liters of TOTAL yellow paint are needed....
We currently have 6 liters of yellow paint, so we need....
10 2/3 - 6 = 4 2/3 MORE yellow paint.
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