AIPMT 2013 Physics and Chemistry Paper

An explosion breaks a rock into three parts in a horizontal plane. Two of them go off at right angles to each other. The first part of mass 1‌kg moves with a speed of 12ms−1 and the second part of mass 2‌kg moves with 8ms−1 speed. If the third part files off with 4ms−1 speed, then its mass is :-