Emily has some marbles. After Mrs. Kim gave her another 21 marbles, the number of marbles became four times the number of marbles at first. How many marbles does Emily have now?

From the beginning of the day the temperature rose 15 degrees so that by 4 pm it was 87 degrees. what was the temperature at the start of the day

Last week , donnell practiced the piano 3 hours longer than marcus . together, marcus and donnell practiced the piano 11 hours . for how many hours did each young man practiced the piano

The diameter of the moon is about 3500 km. approximate the surface area of the moon using the formula for the surface area of a sphere, s = 4πr2. write your answer in scientific notation.