I don't think you can come to the conclusion that sales MUST be down 10%...
there is a piece of information missing here. I'm thinking the margin, and maybe the equation they are working with... I am assuming you are working with the very basic equation of:
Profits = Sales - Cost
Lets try thinking this out loud:
profits = sales - cost
lets say profits last period were 100. Sales 200, Cost 100.
profits down 20% = profits 80 in current period.
Costs up 10% = costs 110 in current period.
X = Sales
80 = X - 110
X = 190 = Sales
That looks like 5% to me...
but look at this scenario:
Profit = Sales - Costs
100 = 1000 - 900
80 = X - 990
X = 1070, Sales go up 7% here.....
but what do I know... derp...