Coding and Decoding · Logical Reasoning and General Mental Ability · UPSC Civil Service

If the symbol $$a + b$$ means $$a - b$$, the symbol $$a - b$$ means $$a \times b$$, the symbol $$a \times b$$ means $$a \div b$$, and the symbol $$a \div b$$ means $$a + b$$, then what is the value of the expression: $$10 + 30 - 100 \times 50 \div 25$$? (All operations are to be replaced simultaneously.)

If in a certain code, 'ABCD' is written as 24 and 'EFGH' is written as 1680, then how is 'IJKL' written in that code?

If in a certain code, 'POT' is written as ATOP and 'TRAP' is written as APART, then how is 'ARENA' written in that code?

In the English alphabet, the first 4 letters are written in opposite order; and the next 4 letters are written in opposite order and so on; and at the end Y and Z are interchanged.

Which will be the fourth letter to the right of the 13^th letter ?