250 x 100 = 25000
250 times 100 is 25000. Because 250 is a quarter of 1000, the answer is a quarter of 100000, which is the quickest way to check it without writing anything down.
250 × 100 = 25,000
Adding 250 to itself 100 times gives the same answer.
Two ways to get there
Add two zeros to 250 and you have 25000. Or take the quarter route: 100 x 1000 is 100000, and a quarter of that is 25000. The quarter trick generalizes, because multiplying by 250 is always the same as multiplying by 1000 and dividing by four.
The ladder of 25
The digits hold still while the decimal point walks. 2.5 x 100 is 250, 25 x 100 is 2500, and 250 x 100 is 25000. Each rung is ten times the one below it, so if your answer has the digits 25 but the wrong number of zeros, you have simply landed on the wrong rung.
Where 25000 lands
250 milliliters a hundred times over is 25000 milliliters, which is 25 liters. 250 grams a hundred times is 25 kilograms. Any per-item figure of 250 across a hundred items gives the same 25000.
About the number
25000 is not a perfect square, despite ending in a square-looking 25. 158 squared is 24964 and 159 squared is 25281, so nothing lands on it. Its only prime factors are 2 and 5.