Cassandra and yvette each have a calculator. yvette starts at a 100 and subtracts 7 each time. isandro starts at zero and adds 3 each time. if they press the keys at the same time, will their displays ever show the same number? if so, what is the number?

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, .