Find the next term of the series 5, 15, 45, 135, ?

Explanation

1. 01

   The differences 10, 30, 90 are not constant, so try ratios: $15 \div 5 = 3$, $45 \div 15 = 3$, $135 \div 45 = 3$.

2. 02

   Each term is 3 times the one before, so the next term is $135 \times 3 = 405$.