Two chemicals a and b are combined to form a chemical c. the rate, or velocity, of the reaction is proportional to the product of the instantaneous amounts of a and b not converted to chemical c. initially, there are 40 grams of a and 50 grams of b, and for each gram of b, 2 grams of a is used. it is observed that 15 grams of c is formed in 9 minutes. how much is formed in 18 minutes? (round your answer to one decimal place.)