Posted by Jane on Monday, April 29, 2013 at 8:51pm.
Alyssa is enrolled in a public-speaking class. Each week she is required to give a speech of greater length than the speech she gave the week before. The table below show the length of several of her speeches:
WK #/Length of Speech (seconds)
3/150, 4/180, 5/210, 6/240
If this trend continues, in which week will she give a 12-minute speech?
By converting 12 minutes to 720 seconds and going week by week, I chose 22.
How do I set this up as a problem and solve it? Thanks
- math - Reiny, Monday, April 29, 2013 at 9:53pm
each week she increases her speech by 30 seconds
12 minutes = 720 seconds
(you are right so far)
This is an arithmetic sequence, where
a = 150 , d = 30
we need to find which term is 720
150 + (n-1)(30) = 720
30n - 30 = 570
30n = 600
n = 20
but what I considered first term is really the third term (3rd week)
so she reaches the 12 minute speech in week 22
You are right!
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