If 8 gal of gasoline costs $27.84, how much would it cost to completely fill a 14-gal tank?

Ineed me i stuck and i don’t want to get it wrong

What is the value of x when x + 7 = 21?

Answer asap ill give ! ’ a sample of radioactive material decays over time. the number of grams, y , of the material remaining x days after the sample is discovered is given by the equation y = 10(0.5)^x. what does the number 10 represents in the equation? a. the half-life of the radioactive material, in days b. the amount of the sample, in grams, that decays each day c. the time, in days, it will take for the samples to decay completely d. the size of the sample, in grams, at the time the material was discovered