On 01/01/1997, Kelly deposits X into a bank account. The account is credited with simple interest at the rate of 10% per year. On the same date, Tara deposits X into a different bank account. The account is credited interest using a force of interest, for t being time in years since the investment is made where 2t/(t^2+k) From the end of the 4 th year until the end of the 8 th year, both accounts earn the same dollar amount of interest. Calculate k.