In a row of boys, Vivek stands thirteenth from the left and Ravi is thirteenth…

For any single seat in a row of people, if that seat is P-th from the left end, it is automatically (Total − P + 1)-th from the right end. Rearranged, this gives the identity: Total = (position from left) + (position from right) − 1 — because that one seat gets counted once from each direction, so the two counts overlap by exactly one.

Applying this to the row of boys:

1. Vivek's original seat is 13th from the left.

2. When Vivek and Ravi swap seats, Ravi moves into Vivek's original seat.

3. The problem states that after the swap, Ravi (now in that seat) is 18th from the right — so Vivek's original seat is 18th from the right.

4. That single seat is therefore both 13th from the left and 18th from the right, so by the identity: Total = 13 + 18 − 1 = 30.

Cross-check with the total of 30: Ravi's original seat was 13th from the right, which by the same identity is 30 − 13 + 1 = 18th from the left. After the swap, Vivek moves into that seat and becomes 18th from the left — consistent with a row of 30 boys.

So there are 30 boys in the row.