A body of mass $M$ at rest explodes into three pieces, in the ratio of masses $1 : 1 : 2$. Two smaller pieces fly off perpendicular to each other with velocities of $30 \mathrm {~m} \mathrm {~s} ^ { - 1 }$ and $40 \mathrm {~m} \mathrm {~s} ^ { - 1 }$ respectively. The velocity of the third piece will be\\
(1) $35 \mathrm {~m} \mathrm {~s} ^ { - 1 }$\\
(2) $50 \mathrm {~m} \mathrm {~s} ^ { - 1 }$\\
(3) $25 \mathrm {~m} \mathrm {~s} ^ { - 1 }$\\
(4) $15 \mathrm {~m} \mathrm {~s} ^ { - 1 }$