Question

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?

Answer 1

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:
$100-7k=3k$
$100=10k \\ 10=k$
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 $100-7*10=30$ and for Cassandra we have, $3*10=30$.