A geometric progression has first term 3 and common ratio 2. How many terms must be added for the sum to be 189?

Using $S_n = \dfrac{a(r^n - 1)}{r - 1}$: $\dfrac{3(2^n - 1)}{1} = 189$, so $2^n - 1 = 63$ and $2^n = 64 = 2^6$, giving $n = 6$. Check: $3 + 6 + 12 + 24 + 48 + 96 = 189$.