Terry opened a savings account in december with $132 and saved $27 each month beginning in january the value of terrys account in july, then calculate the amount and evaluate the reasonableness of your estimate

Let S be the amount in a savings account. 
Let m be the number of months.

General equation
S = 132 + 27m

Specific question
July is the 7th month of the year including both January and July.
S = 132 + 7*27
S = 132 + 189
S = 321

Reasonableness?
Good question. You could estimate an answer.
Start with 150 dollars and say that you add 25 a month.
7 * 25 = 175
S = 150 + 175 = 325 All of this was done in my head. Since the exact answer obtained using the formula is 321, I think that's pretty reasonable using my estimate is a guide.