Find the time between 7 and 8 o'clock when two hands are at right angles?

a)2 2/11 minutes past 7 o'clock
b) 3 2/11 minutes past 7 o'clock
c) 21 9/11 minutes past 7 o'clock
d) 7 10/11 minutes past 7 o'clock

• 
  angle=90 deg
  so,
  angle=30(h)-11/2(min)
  => 90=30(7)-11/2(min)
  =>210-90=11/2(min)
  => (120*2)/11=min
  => min=21 9/11 min
• 8 years agoHelpfull: Yes(9) No(1)
• case 1: degree=(11/2)(min) -30(hr)
  90=(11/2)(min) - 30*7
  min= 54(6/11) past 7 0 'clock
  case2: 60/55 *20
  as it is 20 min apart
  solving we get 21 (9/11) min past 7' 'O clock
  
• 8 years agoHelpfull: Yes(3) No(0)
• Answer is C
• 8 years agoHelpfull: Yes(2) No(1)
• 180=(11/2)-30(7)
  180=11/2-210
  180+210=11/2
  390=11/2
  390*2=11
  780/2=7 10/11 ans
• 8 years agoHelpfull: Yes(0) No(6)