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);