Astack of boards is 21 inches high. each board is 1 3/2 inches thick. how many boards are there?

The number of gallons of 100% mixture to be added is 6.11 gallons and the 10% mixture to be added is 48.89 gallons.

Step-by-step explanation:

Let us assume that x gallons of 10% mixture and y gallons of 100% mixture are added to make 55 gallons of 20% mixture.

So, x + y = 55 ............. (1)

And

⇒ 0.1x + y = 0.2x + 0.2y

⇒ 0.1x = 0.8y

⇒ x = 8y ........... (2)

So, from equation (1) we get, 8y + y = 55

⇒ 9y = 55

⇒ y = 6.11 gallons.

Therefore, the number of gallons of 100% mixture to be added is 6.11 gallons and a 10% mixture to be added is (55 - 6.11) = 48.89 gallons. (Answer)