N = k/ sqrt(t)  where t > 0 and k = constant

N²  = k² / t 

=> t = (k/N)²

old temperature = (k /N )²  = k² / N²

new temperature = (k/2N)²  = k² / 4N²    (when N is doubled)

Δt =  new temperature – old temperature  

     =   (   k² / 4N² –  k² / N² ) 

     =   ( k²  – 4k² ) / 4N² 

     = – 3/4  (k/N)²                                                 

Since it is negative, it is a reduction and converting to % = 3/4 x 100% = 75%