Thanks for that Merenz - I had chosen the right alternative, though as a mathematics teacher for 35 years I am not too keen on using 'by' in this context. What was worrying me was the situation in row 4, column 3, but now I see that this is mentioned.
I am now at the decoding stage which will , I hope, resolve the ambiguity. Thank goodness for Google, however, which both explains an oddity and provides me with the culprit's name. Where would we be without it?