TS Quantitative Reasoning Question

[SxyDentalDiva]

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So I'm doing this QR question on Topscore and I don't understand the explanation they give. Can someone explain this question?

How many gallons of pure orange pulp must be added to 30 gallons of a 20% solution of orange pulp to make it a 50% solution?

 

[NewBee12]

amount of pure orange pulp in the mixture is: x+6
( 6= 30x0.2)
total amount = 30+x
therefore: (x+6)/(30+x) = 0.5
solve for x
x=18
if you need more clarification let me know