Standard formula:

where S is the amount at the end of the time period, P is the initial amount (principle), i = (rate per annum)/(number of periods per year), n = (number of periods per year) (length of investment period).

In your problem, S = 273.53, P = 250, i = (rate per annum)/(1) = rate per annum, n = (1)(2) = 2.

Sub and solve for i.