We want to build a new room that's a bit different from the others. If we had used the usual tiles,we would have needed 25, as always. But, in our room we've already put a big tile which replaces 9 ordinary ones.
How do we find the number of ordinary tiles needed to finish the room?

For 25x9 the blue door;
For 25+9 the yellow door;
For 25-9 the red door.