An XM-1 tank has a mean time between failures (MTBF) of 810 kilometers following an exponential distribution. A mission length of 810 * (ln(0.02)/ln(0.98)) = 980 kilometers would give a 0.98 chance of the tank returning. The probability of a tank returning from a 160 kilometer mission is 0.98. To have a 0.99 probability that at least 5 tanks out of N reach their target 80 kilometers away, 10 tanks should be sent.