The question you opened
Forty eight copies at $12.50
A bookshop sells 48 copies of one title at $12.50 each.
What do the sales come to?
$600
Halve 48 and double 12.50 to get 24 times 25. Halve again for 12 times 50, then once more for 6 times 100, which is 600.
A bookshop sells 48 copies of one title at $12.50 each, which comes to $600. Written down it looks like a job for a calculator. It is two seconds of head work once you know the move. Multiplication does not change if you halve one number and double the other, because you have divided by two and multiplied by two in the same breath. So 48 times 12.50 becomes 24 times 25.
That is already easier. Do it again and you get 12 times 50. Do it once more and you get 6 times 100, which is 600 with nothing left to carry. Three halvings turned an awkward decimal into a number you can read off. The trick works because 12.50 keeps doubling into friendlier shapes: 25, then 50, then 100. Any price ending in 25 or 50 cents behaves the same way, which covers a large share of the prices you meet.
It runs in the other direction too. If one number is odd and the other is even, halve the even one. 35 times 16 becomes 70 times 8, then 140 times 4, then 280 times 2, giving 560. There is a second route worth knowing for this particular sum, because 12.50 is one eighth of 100. So 48 times 12.50 is 48 divided by 8, times 100, which is 6 times 100.
Same 600, arrived at by spotting a fraction instead of halving. Both routes share one habit: look at the numbers before you start, and rearrange them into shapes you already know. Long multiplication is a procedure that works on anything, and that is exactly why it is slow and why decimal points get lost in it. A quick check at the end costs nothing. Fifty copies at roughly twelve dollars would be about 600, so the answer sits where it should.
Technique: Halve one side, double the other