Explanation:

Let the greater number be x and smaller number be y
∴   x – y = 45                       ..... (i)
and   x = 4y                           ..... (ii)
From eqs. (i) and (ii)
4y – y = 45
⇒  y = 45/3 = 15
On putting the value of y in Eq. (ii), we get
x = 4 × 15 = 60
Hence, required sum = x + y = 60 + 15 = 75.