Because of pollution from factories and other activities that release harmful gases into the air
6840 is the answer hope this help
Webb has calculated the percent composition of a compound. He can check his result by adding them to see if they equal up to 100. Why? Well, percent composition tells the quantity of elements with 100 as a base of total amount. This means that it will have to add to 100 to check the result. You would add up all of the values of percent composition of elements to see if they equal 100, and if they do, the results are accurate.
Your final answer: Webb can check his result by seeing if they add up to 100, considering that is the base total quantity.
