You can round numbers based on what number they are closer to.
In example, for the number 33, you would be either rounding to 30 or 40. Since 33 is closer to 30, you would round that to 30.
Between 80 and 90, 89 is closer to 90.
You could then add 90 and 30 to get 120, and then estimate that 33+89 is also around 120.
Hope this helps! :)