How long will it take for the concentration of xy to decrease to 12.5% of its initial concentration when the initial concentration is 0.100?

According to the decomposition of XY is second order, and we assume that the rate constant (K) = 6.92 X 10^-3 M^-1 :

So according to this formula:
Kt= 1/[XY]t - 1/[XY]o

 when [XY]o is the initial concentration = 0.1 m & [XY]t is the concentration after passing a period of time= 0.1 x 0.125 = 0.0125 m 
and when K= 6.92X10^-3 M^-1

 By substitution:

t = (1/[XY]t - 1/[XY]o) / K
  =(1/0.0125 - 1/0.1) / 6.92X10^-3
∴t  = 1011 S = 17 min

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