A school is organizing a charity run. The number of participants doubles every 30 minutes after the registration opens. If 500 people have registered after 2 hours, how many people will have registered after 3.5 hours?

Jawaban: 4000

2 hours = 120 minutes = 4 half-hour periods. 500 = initial × 2⁴ ⇒ initial = 500/16 = 31.25, not integer. To get integer, set initial = 500 at 2 hours? Actually if it doubles every 30 min, after 2h (4 doublings) it is 500, so initial = 500/16 = 31.25. That is not realistic. Better: if after 2 hours there are 500, then after 3.5 hours (1.5 hours later = 3 more doublings) it becomes 500 × 2³ = 4000. So answer 4000. That works regardless of initial. So correct is 4000.