Estimate number of grains of sand in Sarah Desert?

[chenelle00]

The Sarhara Desert covers about 8.3 x 10^13 square feet.
The average depth of the sand in the Sahara Desert is 200ft
A grain of sand has a volume of approximately 1.3 x 10^-9 cubic feet.

I'm told the correct answer is 10^25 but I don't know how to get that answer.

[stapel_eliz]

You are given an area and a depth for the area of sand. Use these to find the volume of sand.

You are given a volume per grain, and have just computed the volume of sand. Divide to find the number of grains.