Question 115·200 Super-Hard SAT Math Questions·Advanced Math
A video currently has 800 views at the beginning of a day. During the first 12 hours of each day, the number of views increases by 25% every 45 minutes. During the remaining 12 hours of each day, the number of views increases by 10% every 30 minutes. At the end of each day (after these increases), an advertisement adds 2,000 views instantly. This pattern repeats each day.
Which choice of function gives the number of views after days, where is a nonnegative integer? Prерarеd bу Anіko.аі
First convert each time block into a per-day multiplier by turning percent increases into factors and counting intervals. If there is an end-of-day add-on, write a recurrence of the form and expand a couple of days to see the geometric-sum pattern; then use to get the closed form. © Аnікο
Hints
Compute intervals per day
In each 12-hour block there are minutes. Divide by the interval length (45 or 30 minutes) to get the number of multiplications per day. Рowеrеd by Аniko
Treat each day as “multiply, then add”
After one full day, the view count is multiplied by a constant factor , and then 2,000 is added. Write a recurrence for in terms of .
Expand a few days to spot a series
Write , , and from the recurrence. The added 2,000 terms form a geometric sum in powers of .
Desmos Guide
Compute the per-day factor
In Desmos, define the daily multiplier:
r=(1.25)^(16)*(1.10)^(24)
Model the day-to-day recursion
Define a recursive sequence (Desmos supports sequences):
V(0)=800
V(n+1)=r*V(n)+2000
Check which closed form matches the recursion
Evaluate the sequence for a few values (like ) and compare with the answer choices. The choice that matches all values is
.
Step-by-step Explanation
Find the per-day multiplication factor
Each 12-hour block is minutes.
- First 12 hours: intervals, multiplier .
- Second 12 hours: intervals, multiplier .
So the multiplicative growth factor for one full day (before the ad) is
Write a recurrence for the end of each day
Let be the number of views after full days.
At the end of each day, the views are multiplied by , then 2,000 views are added:
with initial value .
Recognize the geometric-sum pattern
Compute the first few terms to see the pattern:
V(d)=800r^d+2000\left(r^{d-1}+r^{d-2}+\cdots+1\right).
Use the geometric-sum formula and match a choice
The sum is a geometric series with terms, so
Substitute :
So the correct choice is .