21 x 100 = 2100
21 times 100 is 2100. Multiplying a whole number by 100 adds two zeros, so 21 becomes 2100. Read aloud it is twenty-one hundred, or two thousand one hundred.
21 × 100 = 2,100
Adding 21 to itself 100 times gives the same answer.
The working
Multiplying by 100 is two multiplications by ten: 21 to 210 to 2100. If you would rather break the 21 apart, split it and add the pieces back:
- 20 x 100 = 2000
- 1 x 100 = 100
- 2000 + 100 = 2100
What 2100 divides into
21 is 3 x 7, and the 7 carries through, so 2100 divides by 7 exactly and gives 300. That produces a tidy fact: 2100 days is exactly 300 weeks, because 21 days is exactly 3 weeks and the factor of 100 does not disturb it. In full, 2100 is 4 x 3 x 25 x 7.
2100 read as a time
On a 24-hour clock, 2100 is 9:00 in the evening. Schedules and timetables written in that format are the most common reason people meet the bare number 2100 without any multiplication attached to it.
The usual slip
21 x 10 is 210, one zero short. Since 2100 and 210 both read naturally aloud, the mistake tends to survive a quick reread. Count the zeros in the multiplier and match them.