Another wildfire spread for 7 days and covered a total of 780 square miles. how can you estimate the number of square miles the fire covered per day?

If you want to estimate, first, you should round up the numbers. 7 can be rounded up to 10. 780 can be rounded up to 800. Of course, you can round it in any way you want, but I just rounded it in the way I would do it. Because the total is given in the problem, and it's asking for the number of square miles covered per day, the operation needed for this problem is division. 
So, take your rounded numbers and divide! 800 divided by 10 is 80. The estimation of square miles burned per day is about 80 square miles per day.
Hope this answered your question.