Plz help me understand this question

How many different ways can you make change for $.50 using only nickels,dimes, and quarters?

Which of the fallowing best completes the sequence 5,10,15, a) 30 b) 22 c)25 d)35

Colby and jaquan are growing bacteria in an experiment in a laboratory. colby starts with 50 bacteria in his culture and the number of bacteria doubles every 2 hours. jaquan starts with 80 of a different type of bacteria that doubles every 3 hours. let x equal number of days. colbys experiment follows the model: a.) y=50*2^x b.) y=50*2^8x c.) y=50*2^12x jaquans experiment follows the model: a.)80*2^x b.)80*2^8x c.)80*2^12x