To Find: The sum of all natural numbers from 1 to 100 which are divisible by 4 or 5.
A number divisible by both 4 and 5 should be divisible by 20 which is the LCM of 4 and 5.
Sum of numbers divisible by 4 OR 5 = Sum of numbers divisible by 4 + Sum of numbers divisible by 5 – Sum of numbers divisible by both 4 and 5.