We know that:
rate = 0.15 heaters /labor hour
labourer time = 160 hours / month
goal = 57,600 heaters / year
We can solve this problem by simply looking at which ratios will cancel out the units:
number of labourers = (57,600 heaters/year) / [(0.15 heaters /labor hour) * (160 hours/month) * (12 month/year)
number of labourers = 200