What is the formula for the nth term of a geometric sequence with a4 = 16 and r =1/2 ?

We have a4 = a1 * q^3;
Then, 16 = a1 *  (1/8);
a1 = 128;
The formula for the nth term of a geometric sequence is an = a1 * q^n;
an = 128 * (1/2)^n = (2^7)/(2^n) = 2^(7-n);