In math the sum means to add
She can buy 3 tubes be cause 8.50 time 2 is less then 20$ but 8.50 times three is more than 20$ its 25.5
Answer: Here we have a real life multiplication situation for which an estimate make sense:-
Miley has 212 fiction books. Sharon has 3 times fiction books has many as Miley has, how many fiction books Sharon has
Solution :- Number of fiction books Miley has = 212
Number of fiction books Sharon has= 3 times 212
We know that 212 is near to 200 , so round off 212 to 200 (to the nearest hundred)
Thus the product becomes 
Therefore, the estimated number of fiction books Sharon has=600