Respuesta: N1 + N2 + N3 + ......... + N40
---------------------------------------- = n
40
N1 + N2 + N3 + ......... + N40 = 40n
N1 + N2 + N3 + ....... + N20
-------------------------------------- = n - 9
20
N1 + N2 + N3 + ....... + N20 = 20n - 180
N1 + N2 + N3 + ........ + N60
--------------------------------------- = 12
60
N1 + N2 + N3 + ........ + N60 = 720
20n - 180 + 20n - 180 = 720
40n - 360 = 720
40n = 1080
n = 27
Explicación paso a paso: espero ayudarte