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

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. 

Many things that seem threatening in the dark become welcoming when we shine light on them