Yes, £20. Which if you've done the first one correctly should be close to the right number.
This is a self check, so for example if you'd done half of £36, and accidentally ended up with something that was really off like £5 - to check by rounding, you'd go well OK, £36 is nearly £40 so the answer must be somewhere around £20. Hang on then, £5 can't be right. And you'd know you'd need to go back and calculate half of £36 again.
That is why self-checking is useful in maths, because it can help you spot a mistake. Everyone makes mistakes sometimes, and if you're trying to calculate something important like how much paint you need for a room, it would be a bit of a disaster if you ended up buying too much or too little. Or if you owed somebody money and accidentally gave them not enough, that would be embarrassing.
Rounding something makes it easier, because working out half of £40 is eacher than working out half of £36, but it gives you a figure you can check against and think right, OK, yep, 20 is about 18, I've got it right. Or oh hang on, this is supposed to be about £20 but I've said 12. That can't be right, I'll go back and try again.
Being able to self check gives you confidence in your answers, rather than not really knowing if the answer is correct and just feeling like it's a mystery and guessing game. If they didn't teach this in the past no wonder people think maths is complicated and mysterious! I don't know whether I'm younger than you or just was very lucky with my teachers, but it's one of the most useful skills to give you confidence, IMO.