Sean was playing with his bubble wand. His bubble wand could eject 100 bubbles at once. Half of the bubbles would burst within one minute. 1/4 of the original number of the bubbles would survive for 2 minutes. Only 2% of the original number of bubbles would make it to the end of 3 minutes. All the bubbles would burst at the 4th minute. If Sean blew the bubble wand every minute, how many bubbles were there altogether at the 10th minutes?

Answer 1

so only the last 3 minutes count. All the bubbles in minutes 1-7 have popped.
100 * 1/2 + 100 * 1/4 + 100 * .02 = 77