Question

Find the sum of all natural numbers from 1and 100 which are divisible by 4 or 5.

Answer 1

numbers which are divisible by 4 or 5 =20,40,60,80,100.
a=20 an=100 n=5 d=20
sn=n/2(2a+(n-1)d)
s5=5/2(2×20+(5-1)20)
s5=5/2(40+80)
s5=5/2×120
s5=300...