#### Answer

$20,157$ students are enrolled

#### Work Step by Step

The problem asks for a total. A total is the answer to an addition problem. Therefore, we will add the three numbers (13,786 students enrolled in on-campus day classes, 3497 students enrolled in night classes, and 2874 students enrolled on on-line classes).
$13,786 + 3497 + 2874$
Start at the right of each number and add the ones digits
$6 + 7 + 4 = 17$ ones
Regroup the $17$ ones as $1$ ten and $7$ ones
Write the $7$ ones in the ones column.
Write the $1$ ten in the tens column
Add the tens column next including the regrouped $1$
$1 + 8 + 9 + 7 = 25$ tens
Regroup the $25$ tens as $2$ hundreds and $5$ tens
Write the $5$ tens in the tens column.
Write the $2$ hundreds in the hundreds column
Add the hundreds column next including the regrouped $2$
$2 + 7 + 4 + 8 = 21$ hundreds
Regroup the $21$ hundreds as $2$ thousands and $1$ hundred
Write the $1$ hundred in the hundreds column
Write the $2$ thousands in the thousands column.
Add the thousands column next including the regrouped $2$
$2 + 3 + 3 + 2 = 10$ thousands
Regroup the $10$ thousands as $1$ ten-thousand and $0$ thousands
Write the $0$ thousands in the thousands column.
Write the $1$ ten-thousand in the ten-thousands column
Add the ten-thousands column including the regrouped $1$
$1 + 1 = 2$ ten-thousands
Write the $2$ in the ten-thousands column
$20157$
Separate each group of three digits with a comma.
The sum of the three numbers is $20,157$
$20,157$ students are enrolled.