The answer is 1.98 x 10^10 leaves.
18 mountains x 8800 trees = 158400 total trees
158400 total trees x 125000 leaves = 19800000000 total leaves in entire mountain range
Sci not: 1.98 x 10^10 leaves
To do scientific notation, put the decimal place right after the first digit in the number and count how many places are behind the decimal. you'll write the number of places after the decimal after the ^ in the 10^.
