How many leap years are there from 2001 to 2100, both years included?

Explanation

1. 01

   Multiples of 4 from 2004 to 2100: $(2100 - 2004) \div 4 + 1 = 25$ years.

2. 02

   2100 is a century year not divisible by 400, so it is not leap: $25 - 1 = 24$.