No.

18. Mixture Problems

• Theory
  Weighted Average and Mixture Problems on the GMAT
  Tips and Tricks: Mixtures
  
  • Questions
    DS Questions
    PS Questions

For more check Ultimate GMAT Quantitative Megathread

Hope it helps.
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I'll break down the first question for you.

My personal opinion is that a time line is easier than 4 arrows, but that's just a preference.

(1) Consider a straight line (see image below)


(2) Put the respective ratios on each side of the spectrum

(3) Put 45% between "30" and "50" (the goal of the solution is to mix 30% and 50% and get 45%, right?)

So, 45 is somewhere between 30 and 50. How far is 45 from 30? 15 "steps"

How far is 45 from 50? 5 "steps"

Now, to find the ratio, you simply flip the respective number. 15:5 becomes: 5:15 or simplified to 1:3. So, you use 1 "part" of 30% and 3 "parts" of 45.


Hope this explaination helps

This does not cover simultaneously removal & replacement case .