Which term of the series 4, 9, 19, 40, 79, 159 is incorrect?

Explanation

1. 01

   Test $\times 2 + 1$: $4 \times 2 + 1 = 9$, $9 \times 2 + 1 = 19$.

2. 02

   Next should be $19 \times 2 + 1 = 39$, not 40; then $39 \times 2 + 1 = 79$ and $79 \times 2 + 1 = 159$ both fit.

3. 03

   So 40 is the wrong term.