A particle moving in a straight line covers half the distance with speed $6 \mathrm {~m} / \mathrm { s }$. The other half is covered in two equal time intervals with speeds $9 \mathrm {~m} / \mathrm { s }$ and $15 \mathrm {~m} / \mathrm { s }$ respectively. The average speed of the particle during the motion is :\\
(1) $10 \mathrm {~m} / \mathrm { s }$\\
(2) $8 \mathrm {~m} / \mathrm { s }$\\
(3) $9.2 \mathrm {~m} / \mathrm { s }$\\
(4) $8.8 \mathrm {~m} / \mathrm { s }$