The question illustrates arithmetic operations between fractions and decimals.
1.2 gallons is still in the tank, after 15 days.
From the question, we understand that gallons of water is added to the tank every other day.
So, the total amount added to the tank is:
[tex]\mathbf{Added = \frac{9}{10} + \frac{8}{10} + \frac{7}{10} + \frac{6}{10} + \frac{5}{10} + \frac{4}{10} + \frac{3}{10} + \frac{2}{10}}[/tex]
Take LCM
[tex]\mathbf{Added = \frac{9 + 8 + 7 + 6 + 5 + 3 + 2}{10}}[/tex]
[tex]\mathbf{Added = \frac{40}{10}}[/tex]
[tex]\mathbf{Added = 4}[/tex]
Also, we understand that water is removed from the tank every other day.
So, the total amount removed from the tank is:
[tex]\mathbf{Removed= -\frac{7}{10} - \frac{6}{10} - \frac{5}{10} - \frac{4}{10} - \frac{3}{10} - \frac{2}{10}-\frac{1}{10}}[/tex]
Take LCM
[tex]\mathbf{Removed= \frac{-7 - 6 - 5 - 4 -3 -2 - 1}{10}}[/tex]
[tex]\mathbf{Removed= \frac{-28}{10}}[/tex]
[tex]\mathbf{Removed= -2.8}[/tex]
The amount of water in the tank, after the 15 days is:
[tex]\mathbf{Amount = Added + Removed}[/tex]
This gives
[tex]\mathbf{Amount = 4 -2.8}[/tex]
[tex]\mathbf{Amount = 1.2}[/tex]
Hence, 1.2 gallons is still in the tank, after 15 days.
Read more about fractions and decimals at:
https://brainly.com/question/548650
