Which of the following situations may cause someone to use substitution, in order to solve a system of equations?

a. an exact solution is not needed.
b. one of the equations may easily be solved for a variable.
c. an exact solution is needed.
d. both of the equations are solved for one variable.

Which shows the correct substitution of the values a,b and c from the equation 1=-2x+3x^2+1

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?

How do you write 0.5% as a fraction