The mass of a particular substance is known to grow exponentially at a rate of 0.5% per hour. Its initial mass was 25 grams and, after t hours, it weighed 31 grams.

The equation modelling this growth is

25 x 1.005t = 31

Use the method of taking logs to solve this equation for t, giving your answer correct to the nearest hour.

(Your answer should be a number, without units)