A multiple is the product of the first number and another whole number. Given that 121 is the product of 11 (the first number) and 11, it is a multiple of 11. Therefore, 121 is the correct answer. 

If 6 campers make 3 s'mores sandwiches each, they will make 18 s'mores sandwiches because \(\displaystyle 6\cdot3=18\).

Given that each sandwich requires 2 marshmallows, 36 marshmallows will be required to make the 18 sandwiches, because \(\displaystyle 2\cdot18=36\).

If Arnold writes 5 pages a day and plans to complete his story in 3 days, at which point it will be 45 pages long, he will finish his book by writing \(\displaystyle 3\cdot5=15\) more pages.

This means that Arnold has written 30 pages so far, because  \(\displaystyle 30+15=45\).

If 9 books in a book store have an average of 201 pages, there will be a total of \(\displaystyle 9\cdot201=1,809\) pages among the 9 books.

If Lisa has two snack boxes and each box contains four bags of cookies with three cookies in each, the total number of cookies in the two snack boxes is \(\displaystyle 4\cdot3\cdot2=24\) cookies.

If there are \(\displaystyle 7\) bananas in a bunch and Joe buys \(\displaystyle 8\) bunches, that means he buys \(\displaystyle 56\) bananas because the product of \(\displaystyle 7\) and \(\displaystyle 8\) is \(\displaystyle 56\). However, because a banana is missing from one bunch, the number he buys will be one less, at \(\displaystyle 55\). 

If Jane's cat is sick and she must give it a pill \(\displaystyle 3\) times a day, then she will give it \(\displaystyle 21\) pills in a week. This is because there are \(\displaystyle 7\) days in a week, and the product of \(\displaystyle 3\) and \(\displaystyle 7\) is \(\displaystyle 21\). 

If it takes Rebecca \(\displaystyle 5\) minutes to read a letter and then \(\displaystyle 10\) minutes to respond to a letter, this means that it take her \(\displaystyle 15\) minutes to read and respond to a letter, because the sum of \(\displaystyle 5\) and \(\displaystyle 10\) is \(\displaystyle 15\). 

Therefore, in order for Rebecca to respond to 4 letters, the following equation reveals how many minutes she will need:

\(\displaystyle 4\cdot15=60\)

Thus, \(\displaystyle 60\) is the correct answer. 

To estimate \(\displaystyle 48\cdot21\), the first step is round each number to the nearest tens place. This gives us:

\(\displaystyle 50\cdot20=1,000\)

If Brenda's work is \(\displaystyle 4\) miles away from her home, that means that she rides \(\displaystyle 8\) miles every day that she rides from her home to work (\(\displaystyle 4\) miles), and then from work back to her home (another \(\displaystyle 4\) miles). 

Therefore, the number of miles she rides in \(\displaystyle 5\) days is equal to \(\displaystyle 8\cdot5=40\).