Joyce rode her bicycle 4 miles from her house to the library, and she rode the same distance from the library to the gym. her average speed from the library to the gym was 15 miles per hour less than her average speed from her house to the library. joyce modeled the trip from her house to the gym with the following expression. what does x - 15 represent in this situation?

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?