It costs $ 3.4 to fill the hot tub
<h3><u>Solution:</u></h3>
Given that, You are putting a rectangular hot tub on your back porch
The rectangular hot tub is of shape cuboid
<em><u>The volume of cuboid is given as:</u></em>

It is 6 feet long, 5 feet wide, and 3 feet deep
Therefore,
length = 6 feet
width = 5 feet
height = 3 feet
<em><u>Thus volume of rectangular hot tub is:</u></em>

Thus volume of rectangular tub is 
Convert feet to gallons
We know that,
1 cubic feet = 7.48052 gallon
Therefore,
90 cubic feet = 7.48052 x 90 = 673.247
Thus gallons of water tub can hold is 673.247 gallons
Given that it costs $ 5 per 1000 gallons
1000 gallons = $ 5
Therefore,
1 gallons = $ 
<em><u>Find for 673.247 gallons</u></em>
673.247 gallons = 
Thus it costs $ 3.4 to fill the hot tub