marcodonzelli wrote:
A searchlight on top of the watch-tower makes 3 revolutions per minute. What is the probability that a man appearing near the tower will stay in the dark for at least 5 seconds?
(A) 1/4
(B) 1/3
(C) 1/2
(D) 2/3
(E) 3/4
3 revs / 1 min = 3 revs / 60 sec = 1 rev / 20 sec
if you are standing at the midnight (12:00) marker of a clock and you begin the chronometer,
it will take 20 secs to reach you (1 full rotation);
if you were standing at the three o'clock (3:00) marker and you begin the chronometer,
it would take 5 secs to reach you (1/4 rotation);
if you were standing at the six o'clock (6:00) marker and you begin the chronometer,
it would take 10 secs to reach you (1/2 rotation);
if you were standing at the nine o'clock (9:00) marker and you begin the chronometer,
it would take 15 secs to reach you (3/4 rotation);
so, anywhere you stand past the three o'clock marker, or 3/4 of the clock,
then the chronometer will take more than 5 secs to reach you;
Answer (E)