What is the the value of the fourth term in a geometric sequence if a1=30 and r=1/2?

Jada has a meal in a restaurant she adds up the prices listed on the menu for everything they ordered and gets a subtotal of $42.00. when the check comes, it says they also need to pay $3.99 in sales tax. what percentage of the subtotal is the sales tax

What else equals 42 and 30 for multiply

Which answer choice is it? is it the first ,second,third or fourth giving 40 points for whoever answers correctly you