You can make 75p by adding 34p (100g second class), 33p (we've still got some of the previous rate) and two of the very useful 4p stamps.
Or you can have two 1st (up to 60g) and one 2nd, now 28+28+20 = 76p.
Normally, I'd go "let's not waste 1p, let's go the 75p route, even though I have to lick four stamps rather than stick on three".
Except that we only paid 27p and 19p for the 1st and 2nd class stamps (we bought a pile of them the day before the price went up) so that'd actually cost us 73p. But then we'd have to buy more of them sooner (we're nearly out of the 1sts now).
Argh, decisions decisions :)
Does anyone else have a little spreadsheet to show how to make the various postage prices out of 1st and 2nd class stamps, along with those useful 4p and 1p ones?