The amount, a, that Jane has deposited after t fixed weekly deposits is equal to the initial deposit plus the total amount of money Jane has deposited in the t fixed weekly deposits. This amount a is given to be a=18t+15. The amount she deposited in the t fixed weekly deposits is the amount of the weekly deposit times t; hence, this amount must be given by the term 18t in a=18t+15 (and so Jane must have deposited 18 dollars each week after the initial deposit). Therefore, the amount of Jane’s original deposit, in dollars, is a−18t=15.