The current estimate of the cost of capital is 10%.
But the estimate could be wrong. However it could be anything up to 18.75% and the decision to accept would still have been the right decision.
Therefore it could increase by up to 87.5% of the current estimate before the decision to accept would have been wrong. This is therefore the margin of safety.
Have you watched my free lectures on this? The lectures are a complete free course for Paper F9 and cover everything needed to be able to pass the exam well.