What is the missing letter in this series: m o n n o m p ?

The letter L is missing from the series. In the sequence, the even letters decrease by one each time. The second letter is O. One letter decreased from O is N, which is the fourth letter in this sequence.

The next letter down is M, which is the sixth letter in the sequence. If the sequence continued, the either letter would decrease as well and one letter down from M is L, which is the answer. Logical reasoning is used to answer this question and these type of problems would normally be found on tests such as LSATs and SATs.