What The Heck! Do this in your head. 1000 + 30+1000+10+20+1020+1000+20. You probably got 5000. ( look at bottom)

In my head it is 4100. When I calculate it is 4100. What is the problem?

Same i got 4,100 in my head you calulated that wrong.

when you add up 1000+1000+1000+1020, you get 4020. then add 20,20,30,and 10, which gets you 4100. When you add the smaller numbers you get 100, but since you've been working with thousands in the original question posted above, your mind automatically rewrites it in your head as 5000.