Unwavering is right—.75, or 75%, of an hour is 45 minutes. But the error you're making is a step earlier than that: I'm assuming you're typing 13.25 – 7.5 into the calculator? If so, that won't work because hours/minutes and decimals aren't equivalent. In other words, the 25 in 13h 25min represents 25 out of 60 (the number of minutes in a whole hour), not 25 out of 100 as it does in the number 13.25.
The answer given in the book, 5h 35min, is correct, but to get there you need to subtract 7 and 50/60 from 13 and 25/60. This sounds complex, but you can quickly do it in your head by recognising that 7 hours and 50 minutes is 8 hours, minus 10 minutes. 13h 25min minus 8 hours is 5h 25min, plus 10 minutes gives you 5h 35min.
The upshot of this is that you can't really use the calculator for these kinds of questions: you need to practice quickly adding and subtracting numbers of minutes out of 60. (In addition to the method I've worked through above, you could try visualising a clock face and counting forwards or backwards: the questions nearly all stick to multiples of five minutes.)