Answer:
101+102+103+104+105= 515
Explanation:
the fifth number is counting by 5 each time so you can just count by 5s till you reach 21: 5 (1),10 (2),15 (3),20 (4),25 (5) and so on then once you reach 21 which is 105 you count back from 105 to find the first number then add them all together. This is a simple non-equation solution, I'm sure if I sat there for more than 2 minutes I could think of an equation but it's easier this way, and the problem doesn't require it.