Hi I have a math pproblem can you please help me? its 7th grade stuff?

Question

Suppose one person tells a story to four people in 20 minutes. Then each of those people tell the story to four other people in 20 minutes. How long will it be before 1.25 million people have heard the story?

Answer 1

Wow - now that's a good 7th grade math problem ! !

3 hours 22 minutes 32 seconds

Answer 2

Good question anyway. There is a formula for this but I forget it. So do it the hard way.
2^20 is approximately 1048576 (in fact it is exactly that). Every ten minutes, the number of people who have heard the story doubles. So for 20 doubling times, we need 200 minutes.

Answer 3

To do this problem, multiply. 1 X4 = 20 minutes. then 4 X4 = 16 people in 20 min. 16 X 4 = 64 people in 20 minutes. 64 X4 = 256 people in 20 minutes. 256 X4 = 1,024 people in 20 minutes. 1,024 X4 = 4,096 people in 20 minutes. your now at 2 hours. 4,096 X 4 = 16,384 people in 20 minutes, 16,384 X 4 = 65, 536 people in 20 minutes, 65,536 X 4 = 262,144 people in 20 minutes. Your at 3 hours. 262,144 X 4 = 1,048,576 people in 20 minutes. So in 3 hours and 20 minutes 1,048,576 people would know the story . But you wont it to be know by 1,250,000 people. You have 201,424 more people that need to be told the story. so if the 1,048,576 poeple told 4 more people = 2,097,152. In 3 hours and 40 minutes the story would have been told to more than the required amount of 1,250,000.

Answer 4

x = number of 20 minute periods
4^x = 1.25 million
x = log base 4 of 1.25 million

time = 20 = 20x minutes

Answer 5

I'm not sure what method a grade 7 need to use to solve this, but to solve it algebraically:

the formula of geometric series is:

S=a(r^n-1)/(r-1)

1250000=(4^n-1)/3

3749999=4^n
log3749999=nlog4
n=10.91

10.91*20=218

so approximately 218 minuites.

Answer 6

1250000/20=62500 minutes

Answer 7

Why dont they tell it to everyone at one time? Or put it in a newspaper?

Answer 8

250000 min.