Answer/Step-by-step explanation:
We start with the variable "x". Let's call "x" Dan's full tank.
So we know that right now he has 1/5x or 1/5 of a full tank.
After adding 13 gallons, at the end, he has 7/10x, or 7/10 of a full tank
So we start with
(1/5)x + 13 = 7/10x
Let's subtract 1/5x from both sides
13 = 7/10x - 1/5x
Remember, we need to use common denominators to subtract, so 1/3 becomes 2/10
13 = 7/10-2/10x
Now we simplify to get
13 = 5/10x
Multiply the reciprocal of 5/10 to both sides which is 10/5
13 * 10/5= x
Answer is 26 gallons.
Therefore, Deon's tank can hold 26 gallons.