Complex sentence: In 1937, he visited the Indian Statistical Institute in Calcutta, and its one part-time employee, P. C. Mahalanobis, often returnings to encouraging its development, being the guest of honour at its 25th anniversary in 1957 when it had 2000 employees.
Split into simpler sentences: In 1937, he visited the Indian Statistical Institute in Calcutta, which at the time consisted of one part-time employee, P. C. Mahalanobis. He visited there often in later years, encouraging its development and was the guest of honour at its 25th anniversary in 1957 when it had 2000 employees.