Hi- apologies for the delay in replying. If you fancy another crack at this I have now updated the tests to show the input if you get the wrong answer. Thanks.

Follow the hint and it will become obvious... Otherwise I will have to give the solution:-)

I concur. I can not make heads or tails of this:

"More: e and f must result only from sums (or differences) of products between on the one hand (a, b) and on the other (c, d) each of a, b, c, d taken only once."

What does that mean? Can it be expressed more clearly somehow?

Are you sure you're turning a number and not a string?