The question you opened
Twenty five rows of 48 seats
A cinema has 25 rows with 48 seats in each row.
How many seats does the room hold?
1,200 seats
Twenty five is a hundred divided by four. Multiply 48 by 100 to get 4,800, then take a quarter of it, which is 1,200.
Twenty five is a quarter of a hundred, and both of those are easy. Multiply 48 by 100 to get 4,800, then halve twice: 2,400, then 1,200. The room holds 1,200 seats, and no step required holding a carry. The same idea covers a family of awkward looking multipliers. Times 50 is half of a hundred, so 48 times 50 is 4,800 halved, which is 2,400.
Times 125 is an eighth of a thousand, so 48 times 125 is 48,000 divided by 8, which is 6,000. Times 75 is three quarters of a hundred, so 48 times 75 is 4,800 minus 1,200, which is 3,600. Each one turns into a shift plus one or two halvings. Division works the same way in reverse. Dividing by 25 is multiplying by 4 and moving the point two places, so 1,200 over 25 is 4,800 over 100, which is 48.
Dividing by 50 is doubling and shifting. These come up constantly in percentages, since 25 percent, 50 percent, and 75 percent are the same three fractions. Choosing the route matters more than knowing it. If a factor sits near a power of ten, adjust from the power of ten. If a factor divides a power of ten evenly, use that division.
If neither is true, split the number into round parts and add. Those three moves handle almost every multiplication that appears in a shop, a kitchen, or a spreadsheet. A check worth running here: 25 rows of 48 must sit between 25 times 40, which is 1,000, and 25 times 50, which is 1,250. The answer 1,200 falls inside that bracket, which rules out a lost or added zero.
Technique: Times 25: a quarter of a hundred