To solve this problem you must apply the proccedure shown below:
1. You have the following information given in the problem above:
- Mount Mckinley is 20,320 feet above sea level.
- Mauna kea is 13,796 feet above sea level.
2. Then, you only need to substract both quantities to solve the exercise, as following:

Therefore, as you can see, the answer is: 6,524 feet.
So, how much acid is there in 6 gallons? well is 20% acid or (20/100), so the amount of acid in it just (20/100) * 6 or 1.2, the rest is say water.
now, if we want a 90% solution, and say we add "y" gallons, how much acid is in it? well (90/100) * y, or 0.9y.
now let's add "x" gallons of pure acid, now, pure acid is just pure acid, so is 100% acid, how much acid is there in it? (100/100) * x, or 1x or just x.
we know whatever "x" and "y" amounts are, they -> x + 6 = y
and we also know that x + 1.2 = 0.9y
Hello there.
Explanation:
↓↓↓↓↓↓↓↓↓↓

First, subtract by 4 from both sides.

Second, simplify.

Third, divide by -1 from both sides.

Fourth, simplify.

Next, add by 3 from both sides.

Then simplify.

Therefore, divide by 2 from both sides.

Finally, simplify.

<u><em>Answer⇒⇒⇒⇒x=2</em></u>
Hope this helps!
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-Charlie