How do I answer this math question?

Question

1. The Sahara Desert covers about 8.3 x 10^13 sq ft.

2. The average depth of the sand in the Sahara desert is 200 ft.

3. A grain of sand has a volume of approximately 1.3 x 10^-9 cubic feet.

Which of the following is the best number of grains of sand in the Sahara desert?

A. 10^22

B. 10^23

C. 10^24

D. 10^25

I am studying to take my content exam to become a teacher, and I have NO idea how to come to this answer...I know the answer (from the answer guide), I just need to know HOW to get to the right answer....Thanks in Advance

SOURCE: TEXES preparation guide online

Answer 1

answer d

volume of dessert = 8.3 x 10^13 sq ft  *  200 ft.  =  1.66 E 16 cubic feet

volume of 1 grain sand = 1.3 x 10^-9 cubic feet

grains of sand in the Sahara desert = 1.66 E 16 cubic feet / 1.3 x 10^-9 cubic feet     =    1.27692 E 25   =  10 ^ 25  = ans d

Answer 2

find volume of sahara desert then divide that by the volume of 1 grain of sand.

Answer 3

This question is just like you have a bucket of water of a given volume and how many glass(of a given volume, grains in this case) can be filled from that bucket(desert in this case).

You are given the area of the desert that is length x breadth.

area=8.3x10^13.

volume of a cuboid=length x breadth x depth.

therefore total volume of the desert=(length x breadth) x depth.

                                                 = area x depth

                                                 = 8.3x10^13 x 200

                                                 =1660 x 10^13 or

dividing the total volume by the volume of 1 grain gives you the number of grains

number of grains= total volume/volume of 1 grain

                        = 1.66 x 10^16/1.3 x 10^-9

                        =1.27 x 10^25 (approximately)

therefore answers is D..