SOLUTION: At what time between 7pm and 8pm will the minute hand be 90 degrees behind the hour hand?

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Question 567470: At what time between 7pm and 8pm will the minute hand be 90 degrees behind the hour hand?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Each hour mark on the clock is +5%2F60+=+1%2F12+ of
a +360+ degree circle apart. That is +%281%2F12%29%2A360+=+30+ degrees
At 7 pm, the minute hand and hour hand are
+7%2A30+=+210+ degrees apart
The minute hand is moving at the rate of +360+%2F+60+=+6+ degrees/min
The hour hand is moving at the rate of +30%2F60+=+1%2F2+ degree/min
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Let +d%5B1%5D+ = the degrees that the minute hand travels until it makes
an angle of +90+ degrees with the hour hand
Let +d%5B2%5D+ = the degrees that the hour hand travels until it makes
an angle of +90+ degrees with the minute hand
Let +t+ = time in minutes until they are +90+ degrees apart
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(1) +210+%2B+d%5B2%5D+-+d%5B1%5D+=+90+
(2) +6t++=+d%5B1%5D+
(3) +%281%2F2%29%2At+=+d%5B2%5D+
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This is 3 equations and 3 unknowns, so it's solvable
Substitute (2) and (3) into (1)
(1) +210+%2B+%281%2F2%29%2At+-+6t+=+90+
(1) +420+%2B+t+-+12t+=+180+
(1) +-11t+=+180+-+420+
(1) +11t+=+240+
(1) +t+=+21.818+ min
+.818%2A60+=+49.09+ sec
The hands will be 90 degrees apart at 21 min and 49 sec past 7 PM
hope I got it