To represent the total cost \( C \) of Joseph's cellphone data plan using the variable \( m \) for the number of months, you can write the expression for the total cost as:
\[ C = 21 + 55m \]
Where:
- \( 21 \) is the one-time startup fee,
- \( 55m \) is the monthly charge for \( m \) months.
To find the total cost for 6 months, substitute \( m \) with 6 in the expression:
\[ C = 21 + 55(6) \] \[ C = 21 + 330 \] \[ C = 351 \]
Thus, the total cost for 6 months is $351.
So, the entire response can be summarized as follows:
Expression: \[ C = 21 + 55m \]
The total cost for 6 months is $351.