Use sigma notation to write the sum

20 + 25 + 30 + 35 + ... + 100.

Answer

All the terms here are multiples of 5.

20 + 25 + 30 + 35 + ... + 100 = 5(4) + 5(5) + ... + 5(20).

If the general term is 5n, then the series should start at n = 4 and stop at n = 20.