Question

Jumlah kelipatan 5 antara 20 dan 80

Answer 1

20,25,30,...80
Un = bn + (a-b)
80  = 5n + 15
65  = 5n
n    = 13

S13 = (a+Un) x n/2
       = (20+80) x 13/2
       = 100 x 13/2
       = 650

Answer 2

Mis :
b = bilangan terakhir
a = bilangan awal
k = kelipatan berapa

maka
sn = {(b-a /k) + 1} × { b+a/2}
sn = {(80-20/5) +1} × {80+20/2}
sn = {(60/5) +1} × {100/2}
sn = {12+1} × {50}
sn = {13} × {50}
sn = 650