Hey there! I'm happy to help!
We want to find the volume of this rectangular waterbed. This means the amount of space it takes up. To find the volume of a rectangular prism, you just multiply together the three side lengths.
7×5×1=35 cubic feet
Now, we need to see how many gallons fit into 35 cubic feet. We see that one gallon is equal to 0.13 cubic feet. So, we can set up a proportion to find how many gallons are needed. We will use g to represent our missing number of gallons.

In a proportion, the products of the diagonal numbers are equal. This means that 35, which is 1 multiplied by 35, is equal to 0.13g, which is from multiplying 0.13 by the g.
0.13g=35
We divide both sides by 0.13/
g≈269.23
When rounded to the nearest whole gallon, we will need 269 gallons of water to fill the waterbed.
I hope that this helps! Have a wonderful day! :D