quiz of the day: bridge-crossing

A group of four persons, called P1, P2, P3 and P4, have to cross a bridge at night. It is dark and they can only cross the bridge if they carry a lamp. Only one lamp is available and at most two persons can cross at the same time. Therefore any solution requires that, after the first two persons cross the bridge, one of them returns, bringing back the lamp with him for the remaining people. The four persons have different maximal speeds: here P1 crosses in 5 time units (t.u.), P2 in 10 t.u., P3 in 20 t.u. and P4 in 25 t.u. When a pair crosses the bridge, they move at the speed of the slowest person in the pair. how could u move the whole group to the other side of the bridge in 60 t.u?