Answer:
The missing number is 4
Step-by-step explanation:
Given
21 x 39 x _____ x 19 divided by 12 = 5187
Required
Fill in the gap
Let the missing number be represented with y; So, the expression becomes

Multiply both sides by 12


Rearrange the expression on the left hand side


Divide both sides by 15561



Hence, the missing number is 4
Answer: Yes
Step-by-step explanation:
First multiply the first equation by three to get 3x+12y=69
Then subtract 12y from both sides of the second equation
Then add the first system and the second like this:

Because 0≠70, the system has no solutions
She will need 6 boxes so she will have 6 more boxes left over