Julia pays $56.70 to fill her car with 15 gallons of gas.
which equation can be used to find c, the cost per gallon?

56.70/15 = 3.78

The cost per gallon, c, is equal to 3.78

This is a half life problem.
you'll find this page quite but i'll solve your question anyway.
time = half-life * log [bgng amt / ending amt] / log 2
time = 5,600 * log [100 / 15] / log 2
time = 5,600 * 0.82390874095 / 0.30102999566
time = 5,600 * 2.7369655942
time = 15,327 years