This can be be solved the following way:
Yvette starts at 100 and subtracts 7 k times, as for Cassandra (or Isandro, the question is not clear on this) she starts at 0 and presses adds 3 k times. This leads to the following equation:
So for k=10 they will both display the same number.
We can now either choose the viewpoint of Cassandra or Yvette:
If we chose Yvette then we'll have
and for Cassandra we have, 
.
Yvette starts at 100 and subtracts 7 k times, as for Cassandra (or Isandro, the question is not clear on this) she starts at 0 and presses adds 3 k times. This leads to the following equation:
So for k=10 they will both display the same number.
We can now either choose the viewpoint of Cassandra or Yvette:
If we chose Yvette then we'll have