h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Here is a variation of scrabble that will fry your minds.

Replace the letter tiles with numbers. There are three main variations 10 sets of tiles (1-10) 4 sets of tile (1-25) and finally 1 set of tiles (1-100).

The idea is make an equation ie 1+5=6. to achieve this you place down the tiles 1 5 6. The fun part is that you can reuse the tiles already on the board. For example 1 5 6 can be rephrased as 1-5=-4+6=2+3=5. The tiles placed down are 1 5 6 3 5. The last tile placed down must be the answer.

You can score the game the same way as in normal scrabble. The only extra rule is that if challanged you must show how you worked it out.
How do I get the answer 79 from the following 7 11 25 30 3 75 (using that order).

An important aspect of this game is the use of your imagination

I wrote to Spear and Jackson (British producers of scrabble) with this variation. But I got the response saying that is was unmarketable.