When a person breathes carbon monoxide (CO), it enters the bloodstream to form carboxyhemoglobin (COHb), which reduces the transport of oxygen to tissues. The formula given by T(x) = 0.0079x² - 1.53x + 76 approximates the number of hours T that it takes for a person's blood- stream to reach the 5% COHb level, where x is the concentration of CO in the air in parts per million (ppm) and 50 ≤  x ≤ 100. (Smokers routinely have a 5% concentration.) Estimate the CO concentration x necessary for a person to reach the 5% COHb level in 4-5 hours.