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How many times are an hour hand and a minute hand of a clock at right angles during their motion from 1.00 p.m. to 10.00 p.m. ?
In an hour once will be at right angle when hour hand is just forward to 1 and minute hand is forward to 4. Secondly will be at right angle when hour hand is near to 2 and minute hand is near to 11.
∴ Required No. of times will be = 18.