100 x 15 = 1500
100 times 15 is 1500. The quickest route is ten times the number plus half of that again, because fifteen is ten plus five and five is half of ten. It works on any number, not just this one.
100 × 15 = 1,500
Adding 100 to itself 15 times gives the same answer.
Ten times, then half again
Fifteen is ten plus five, and five is half of ten. So anything times 15 is that number times ten, plus half of the result. Here 100 x 10 gives 1000, half of 1000 is 500, and 1000 + 500 is 1500. The method is not special to round hundreds: 24 x 15 is 240 plus 120, which is 360.
Same digits, different question
- 100 x 15 = 1500
- 15 percent of 100 = 15
- 15 percent of 10000 = 1500, the version that does match this answer
- 100 + 15 = 115
A percentage multiplies and then divides by 100, so a fifteen percent figure only reaches 1500 when the base is 10000.
1500 on its own
Its prime factors are 2 x 2 x 3 x 5 x 5 x 5, which gives it 24 whole divisors and makes it easy to break up: 3 shares of 500, 4 of 375, 5 of 300, 10 of 150, 12 of 125. It is not a square number, since 38 squared is 1444 and 39 squared is 1521. Outside arithmetic the same four digits mean other things. On a 24-hour clock 1500 is three in the afternoon, and on a track the 1500 metres is the race nicknamed the metric mile.
Near neighbours
100 x 12 is 1200 and uses the same shape of trick, ten times plus two times. 1.5 x 100 is 150, exactly a tenth of this answer and a common mistype of it. The rest are indexed at multiplication answers.