A hollow circular column of internal diameter 20 mm and external diameter 40 mm has a total length of 5m. One end of the column is fixed, and the other end is hinged. Find out the crippling stress of the column if E = 2×10⁵N/mm^2. Also, find out the shortest length of this column for which Euler’s formula is valid taking the yield stress equal to 250 N/mm²