17. A certain sum doubles in 7 years under simple interest. In how many years will the sum become four times itself?
a) 14
b) 21
c) 28
d) NOTA

• this can be solved without writing anything .Think like
  Doubles in 7 years
  Trebles in 14 years
  4 times in 21 years
• 9 years agoHelpfull: Yes(10) No(2)
• sum double for in 7 years mean=2*(7years)=14years equivalent to 4times
• 9 years agoHelpfull: Yes(4) No(5)
• ans=21
  
  
  
  
  
  
• 9 years agoHelpfull: Yes(2) No(0)
• Ans 14 yrs
  2p=p*r*7/100
  r=200/7
  4p=p*(200/7)*t/100
  t=14
• 9 years agoHelpfull: Yes(2) No(2)
• principle Amount Time
  x --------> 2x(double) ------>7 years ----------->1st period
  2x--------> 4x ------>7 years ----------> 2nd perios
  _________
  14 years
  shortcut: 2x ------------> 7years
  4x-------------> ?
  convert 4 interms of 2(because sum is double)--------->2^2
  multiplay power 2 with time 7 ----------->7*2=14
  Ans:14
• 9 years agoHelpfull: Yes(1) No(8)
• Answer is 21 yrs only
  
• 9 years agoHelpfull: Yes(1) No(0)
• let sum be x,
  x->2x (in 7 years) so; clearly S.I=x
  (x*7*r)/100=x
  or, r=100/7
  
  now x->4x (in t yrs) clearly here S.I=3x
  so (x*t*100)/(7*100)=3x
  or, t=21 yrs
• 5 years agoHelpfull: Yes(0) No(0)