Marco starts with a population of 10 bacteria that doubles every day for a number of days, d. He writes the expression 10(2d) to find the number of bacteria after d days. Isabella starts with a population of 1 bacterium that increases 20% every day for a number of days, d. She writes the expression (1 + 0.2)d to find the number of bacteria after d days.

Use the drop-down menus to explain what each part of Marco’s and Isabella’s expressions mean.