Gentlemen re-start your engines!

3 wires from the outside come inside to the breaker box.

A 100 AMP double breaker labeled "MAIN" receives the 2 hot wires.

The neutral is wired to ground.

The output of the 2 MAIN breakers (when switched ON) goes to 2
vertical busses.

We are examining the left (odd) side column of breakers; 13 and 15.

Question 1. Are 13 and 15 on the same leg? (I assume so)

Question 2. What should the black of 15 measure to a lifted white
wire?