a child's record player special records called"40 rpm," maening the record rotates 40 times per minute.

Question

how long does it take one of these records to rotate once?

Answer 1

If it makes 40 rotations in a minute, it makes 1 rotation in 1/40 of a minute.

1/40 minute * 60 seconds/minute = 60/40 seconds
60/40 seconds = 1.5 seconds.

You can double check your answer... what is 1.5 sec x 40 rotations? 1.5 x 40 = 60 seconds. So it takes a minute to make 40 rotations (40 rpm). This proves that 1.5 is correct.

You can also do this algebraically:

40 rotations / 60 seconds = 1 rotation / n seconds.
40 / 60 = 1 / n
40n = 60
n = 60/40 = 1.5 seconds

So the answer is 1½ seconds per rotation.

Answer 2

90 seconds

40 turns per minute:
60 Secs for 40 turns

60/40 = 1.5 (mins) => 90 secs

Answer 3

4 seconds

Answer 4

40 rev/60 sec = 1 rev / x sec

x = 1.5 seconds