Answer:
5
Step-by-step explanation:
According to the described rule, we have

We can see the pattern

In other words, for all 

Now,

The pyramid is shown in the diagram below.
The pyramid is built from four congruent triangles and one square as the base
We have the side of the square, so the area is = 10×10 = 100
We need the height of the triangle to work out its area. We can find out by using the height of the pyramid and half of the length of the side of the square.
Using the Pythagoras rule
Height of triangle =

Area of one triangle = 1/2×10×12=60
Surface area of the pyramid = 100 + (4×60) = 340 square inches
Hmmm... a geometric sequence MUST have a fixed common ratio. If it is changing, then the sequence you are looking at might not be a geometric sequence at all. We'd need to see an example to be sure.
60 / 5 = 12 miles per gallon
36 miles = 12 miles / gal * x gallons
divide by 12
36/12 =x gallons
3 =x
He needs 3 gallons
Looks like this
Everything in one picture