def conditions(i):
if i <=50: return '0-50'
if i > 50 and i <=100: return '50-100'
if i > 100 and i <=250: return '100-250'
if i > 250 and i <=350: return '250-350'
if i > 350: return '>350'
df['C']=df['B'].apply(conditions)