A row boat moves along the surface of a calm lake at $1.2 \space m/s$ toward a nearby dock. A girl of mass $m_1 =45 \space kg$ is sitting in the boat when her friend of a mass of $m_2 = 75 \space kg$ runs along the dock and jumps into the boat.
He lands with a horizontal velocity of $3.5 \space m/s$ in the direction opposite to the boat's original motion.
What is the boat's mass if its final velocity is $0.95 \space m/s$ in the direction opposite to its original motion? Assume that both the girl and her friend remain in the boat.