Concept: The question tests on basics of Lcm/hcf
Solution:
The product of 2 numbers is equal to the product of their LCM and HCF
Given that 1 number is 75. Let the other number = b
Therefore 75 * b = LCM * HCF .. (1)
Given that LCM + HCF = 400 and that LCM = 15 HCF
SO 16 HCF = 400 or HCF = 25
LCM = 15 * 25 = 375
Putting in (1) 75 * b = 25 * 375
b = 125
Option E