Adults and children can sign up for a virtual fun run race. It costs $30 per adult and $20 per child for the fun run race. The expression 30a + 20c can be used to calculate the cost (in dollars) for adults and c children to participate in the virtual fun run. Use the expression to determine the total cost (in dollars) for 2 adults and 4 children to participate in the fun run.

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?