Answer: The required number of extra gallons needed is 5.125.
Step-by-step explanation: Given that a 20.5 gallon fish tank is [tex]\dfrac{4}{5}[/tex] full.
We are to find the number of more gallons that are needed to fill the tank.
We will be using the UNITARY method to solve the problem.
We have
Number of gallons needed to fill [tex]\dfrac{4}{5}[/tex] of the tank = 20.5.
So, the number of gallons needed to fill the complete tank is given by
[tex]\dfrac{20.5}{\frac{4}{5}}=20.5\times\dfrac{5}{4}=25.625.[/tex]
Therefore, the extra gallons needed to fill the tank is
[tex]25.625-20.5=5.125.[/tex]
Thus, the required number of extra gallons needed is 5.125.
