The margin of safety is the difference between the total sales and break-even sales. It may be expressed in monetary terms or as a percentage, i.e., the margin of safety in relation to total sales. It is an extremely valuable guide and indicates the financial strength of the business.

Margin of safety = profit / PV ratio

So according to your quoted details-

Margin of safety = 18000/50

= 360

Which is desired answer.

I hope this helps.
Regards
Madhur