Divide the stake by the number to the right of the / - in this case 5 - and multiply by the number to the left - 7 - so they'd have won £140 and got their £100 stake back if Plymouth won.
But look at the teams they've lost or drawn to away in that time. Us, Peterborough, Bolton, Ipswich, Wednesday (pre-blip). Not many home defeats for any of those teams.
I was on the last train out of Paddington to Plymouth last night. Lots of sour faces with destroyed expectations.