Answer:
10%
Step-by-step explanation:
Given that,
Interest at last year debt = 8%
Current year cost of debt = 25% higher
Firms paid for debt last year = 10%
Firms paid for debt in current year = 12.50%
 Kd - cost of debt
 Yield = Interest at last year debt × (1 + increase in cost of debt)
 = 8% × (1 + 0.25) 
 = 8% × 1.25
 = 10%
Kd = Yield (1 – T) 
 Kd = 10% (1 – 0) 
 = 10% (1) 
 = 10%
Therefore, after tax cost of debt would be 10%.