An artisan starts working at $08 \mathrm{~h}$ and produces six bracelets every 20 minutes; his assistant starts working one hour later and produces eight bracelets of the same type every half hour. The artisan stops working at $12 \mathrm{~h}$, but tells his assistant that he should continue working until he produces the same number of bracelets as him. At what time will the assistant stop working?
(a) $12 \mathrm{~h}$
(b) $12 \mathrm{~h} 30 \mathrm{~min}$
(c) $13 \mathrm{~h}$
(d) $13 \mathrm{~h} 30 \mathrm{~min}$
(e) $14 \mathrm{~h} 30 \mathrm{~min}$
The correct option is (d).

The craftsman produces 6 bracelets every 20 minutes. Since 1 hour = 60 minutes = 3 × 20 minutes, the craftsman produces 6 × 3 = 18 bracelets in one hour. Since he worked 12 hours - 8 hours = 4 hours, the number of bracelets made by the craftsman is 18 × 4 = 72. The assistant produces 8 bracelets every half hour, so in 1 hour he produces 16 bracelets. To produce 72 bracelets, he will need 72 / 16 = 4.5 hours = 4 hours and 30 minutes. Since he starts his work at 9 o'clock, he will finish his work at 9 + 4.5 = 13:30.