If the average human colon weighs 220 grams, how many bacteria live in each person's colon? if there are 6.7 billion people on earth, what is the total number of bacteria that live in human colons around the world? answer

It is estimated that between 10¹⁰ and 10¹² bacteria per 1 gram live in the colon of an average person.
To answer this question, assume that there are 10¹² bacteria per gram of human colon. 

Because the average human colon weighs 220 grams, the number of bacteria in the colon is
(220 g)*(10¹² bacteria/g) =  2.2 x 10¹⁴ bacteria.

There are about 6.7 billion (6.7 x 10⁹) people on earth. Therefore the total number of bacteria in human colons is about
(2.2 x 10¹⁴ bacteria/person)*(6.7 x 10⁹ people) =  1.424 x 10²⁴ bacteria.

About 2.2 x 10¹⁴ bacteria per person.
About 1.424 x 10²⁴ bacteria in human colons.

Note:
These answers use the upper estimate of 10¹² bacteria per gram of human colon. The lower estimate is 10¹⁰ bacteria per gram of human colon.