8th Grade Functions: Battery Life
RULE
they planned to
Sam wants to take his tablet and phone on a car trip. An hour before they planne
leave, he realized that he forgot to charge the batteries last night. At that point
plugged in both devices so they can charge as long as possible before they left.
Sam knows that his tablet has 40% of its battery life left and that the battery charge
by an additional 12 percentage points every 15 minutes.
His phone is new, so Sam doesn't know how fast it is charging but he recorded the
battery charge for the first 30 minutes after he plugged it in. (see the table below)
Time charging (minutes)
10
20
30
Phone battery charge (%)
20
/
32
44
56
a. If Sam's family leaves as planned, what percent of the battery will be charged for
each of the two devices when they leave?!
b.
How much time would Sam need to charge the battery 100% on both devices?

My son is named Sam and this is a pretty accurate account of what he's like before a trip.

Sam has an hour to charge things.

His tablet is at 40% and charges 12 points every 15 minutes.  So four times that or 48 points in a hour. So after the hour his tablet is at 40+48=88%.

His phone went from 20% to 56% in 30 minutes, i.e. 36 points in 30 minutes.  We can expect another 36 points for the remaining half hour, so after the hour his phone is at 56+36=92%

a.  Tablet 88%, Phone 92%

b.  To get a full charge on the tablet Sam needs to add 60 points at the rate of 12 points per 15 minutes, 60/12=5 15 minute intervals, which is 75 minutes.

To get a full charge on the phone, it starts at 20% so needs to get 80 points at the rate of 36 points per 30 minutes.  So the total time is

(80/36)*30 = 200/3 minutes, which is 66 2/3 minutes.

Tablet: 75 minutes  Phone: 66 2/3 minutes

correct answers below:

the answer is y < -3 or   y > 3

-3y > 7 + 2

-3 y > 9

y < -3