June 2013 Q4a

Budgeted production costs of housing = 6902 + 1302 - 575 = 7629 (better to base sales and profits on budgeted costs so that there is incentive to control actual costs) Applying a 30% (somewhat arbitrary) mark-up to that gives 1.3 x 7629 = 9918.

why need to apply 30%? why cant apply 10% or 20% instead of 30%?

You could. The answer makes it clear that this, perhaps, is only a typical markup. Othrrs would workm but 10% might be a bit low.