Assume the chemical reaction converts chemicals A and B into the chemical C, with rate of reaction directly proportional to product of amounts of chemicals A and B remaining. It is known that 2 grams of chemical B are utilized in the reaction for every gram of chemical A. If there initially are 60 grams of chemical A and 90 grams of chemical B and no chemical C, write the differential equation explaining amount of chemical C which will be present minutes after reaction starts.