How come it took her 48 minutes?

that's because you are forgetting that once you go up to 11.59 in the time scale, the next number is 12.00 and NOT 11.60!!! you are assuming that after 11.59, you get 11.60, 11.61 and so on until 11.99.!! you can't substract time the way you do substract ordinary numbers with two decimals.