KPMG的一条笔试题

What was the loss in potential sales revenue attributable to the production of sub-standard (as opposed to standard) buttons over the 6 month period?
(a) 13.75
(b) 27.50
(c) 137.50
(d) 280.00
(e) 275.00

我怎末都算不出答案，请大家帮忙看下，到底咋算的呢？