A steamer going upstream would cover the distance between town A and town B in 4 hours and 30 minutes. If the same steamer going downstream would cover the distance between the towns in 3 hours, how long would it take a raft moving at the speed of the current to float from town B to town A?

Amichelson interferometer operating at a 600 nm wavelength has a 3.02-cm-long glass cell in one arm. to begin, the air is pumped out of the cell and mirror m2 is adjusted to produce a bright spot at the center of the interference pattern. then a valve is opened and air is slowly admitted into the cell. the index of refraction of air at 1.00 atm pressure is 1.00028.how many bright-dark-bright fringe shifts are observed as the cell fills with air?