Question

Estimate how many times larger 6 x 1020 is than 29 x 1016.

Answer 1

The larger one should be 29*1016. Round 1016 down to 1000.
You get 29000 when you do that.
6 * 1020 is roundable to 6*1000 which is 6000

If you round 29000 to 30000 then 29*1016 should be 5 times larger than 6 * 1020

Check it out.
29 * 1016 = 29464
6 * 1020 = 6120

When you divide the smaller into the larger, you get 4.18 which is in the same ball park as 5. 

Estimating is a skill you need when you are shopping and when you are writing tests. If you need to know if 4.18 is a good answer when a rounded answer is 5, you know you've done the work properly.

Rounding should permit you to get an answer in your head.