Speed reduced to half by 15 people implies 15 people make half of the work group, making the total strength = **30**

N men can do 1 work in 30 days

(N - 15) men can do 1 work in 60 days

Speed of N men = 1 work / 30 days

Speed of (N - 15) = 1 work / 60 days

Speed of 1 man = 1 work / (30×N days) = 1 work / (60×(N - 15) days)

30N = 60N - 900

30N = 900

N = 30 men.