How many times are the hour hand and the minute hand of a clock at right angles during their motion from 1.00 p.m. to 10.00 p.m.?
(a)9
(b)10
(c)18
(d)20
Answer
Answer (as printed): C
Explanation
The duration from 1.00 p.m. to 10.00 p.m. is 9 hours and during each of these 9 hours the hands of the clock are at right angles twice. So, required number $=9 \times 2=18$.
Explanation as extracted from the printed page; notation may be imperfect.