For a closed pipe, only odd harmonics are present. first overtone frequency is 3fo_oo, 2nd^{\text{nd}}nd overtone frequency is 5fo_oo etc.
4th^{\text{th}}th overtone frequency = 9fo_oo
900 = 9fo_oo
fo_oo = 9009\;\frac{900}{9}9900 = 100Hz