Letter and Symbol Series - Verbal and Reasoning

Directions to SolveIn these series, you will be looking at both the letter pattern and the number pattern. Fill the blank in the middle of the series or end of the series.
Q1:

DEF, DEF2, DE2F2, _____, D2E2F3

A DEF3

B D3EF3

C D2E3F

D D2E2F2

ANS:D - D2E2F2

Explanation: In this series, the letters remain the same: DEF. The subscript numbers follow this series: 111, 112, 122, 222, 223, 233, 333, ...