Respuesta :

for this example you can simply substitute i with numbers 1,2,3,4,5, and add terms
4*1+4*2+4*3+4*4+4*5 =60

for more difficult cases you can use formula
(a1+an)n/2

a1=4,  a5=20, n=5
(4+20)*5/2=24*5/2=12*5=60