**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