7 A particle \(P _ { 1 }\) is projected vertically upwards, from horizontal ground, with a speed of \(30 \mathrm {~m} \mathrm {~s} ^ { - 1 }\). At the same instant another particle \(P _ { 2 }\) is projected vertically upwards from the top of a tower of height 25 m , with a speed of \(10 \mathrm {~m} \mathrm {~s} ^ { - 1 }\). Find

1. the time for which \(P _ { 1 }\) is higher than the top of the tower,
2. the velocities of the particles at the instant when the particles are at the same height,
3. the time for which \(P _ { 1 }\) is higher than \(P _ { 2 }\) and is moving upwards.