Johnny and his father go fishing at 5 AM. After motoring 6km upstream, Johnny writes a letter to his friend, puts the letter in a bottle. and drops it in the river. The bottle floats in the current and reaches 7:40 AM to the point where Johnny and his father began their trip. If the speed of the boat in still water Is 9 km/hr, what is the speed of the river current?

Aspacecraft can attain a stable orbit 300 kilometers above earth if it reaches a velocity of 7.7 kilometers per second. the formula for a rocket's maximum velocity v in kilometers per second is vequalsminus0.0098tplusc ln upper r, where t is the firing time in seconds, c is the velocity of the exhaust in kilometers per second, and r is the ratio of the mass of the rocket filled with fuel to the mass of the rocket without fuel. find the velocity of a spacecraft whose booster rocket has a mass ratio of 20, an exhaust velocity of 2.1 km/s, and a firing time of 15 s. can the spacecraft achieve a stable orbit 300 km above earth?