High concentrations of carbon monoxide co can cause coma and possible death. the time required for a person to reach a cohb level capable of causing a coma can be approximated by the quadratic model tequals .0002 squared minus.316 plus127.9, where t is the exposure time in hours necessary to reach this level and 500less than or equals xless than or equals 800 is the amount of carbon monoxide present in the air in parts per million (ppm). a) what is the exposure time when xequals 750 ?