Correct Answer - Option 3 : 3

**Explanation:**

There are 3 arithmetical checks in rise and fall method: ∑BS - ∑FS = ∑Rise - ∑Fall = Last RL – First RL. Since there is complete check on RLs of intermediate stations, hence errors, if any, in the intermediate sights are eliminated.

Arithmetic checks applied on readings are only meant for the accuracy of calculations i.e. correctness of computations. It does not verify the accuracy of fieldwork.

i. Arithmetic check for Collimation Method:

Sum of B.S. – Sum of F.S. = last R.L. – First R.L.

ii. Arithmetic check for Rise and Fall Method:

Sum of B.S. – Sum of F. S. = Sum of rise – Sum of fall = last R. L. – first R.L.