Estimate means to round so you could round to the nearest ten, hundred, thousand, and ten thousand. Which place do you need to round to?
If four croissants cost $2.60, you divide 2.60/4 and get .65.
That means that one croissant costs .65 cents.
If you multiply .65 x 7, you get $4.55.
It costs $4.55 to purchase 7 croissants.
10, because 6(2) is 12 then you divide that by 3 which is four to which you add 6. therefore the answer is 10.