John borrowed $80 from his sister, Joan. He agreed to pay her back at a rate of $6 per week. Create a function, D, that gives the amount of money that john still owes Joan after
w weeks.
(b) John told Joan that he will have paid her back all of the money, after making
payments for 13 weeks.. Explain why John is incorrect?

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?