Let the numbers (in decreasing order) be a,b,c,d,e,f Average of first 5 = 30 ⇒ Sum of first 5 numbers = 30×5 ⇒ (a+b+c+d+e)=150 ------(i) Similarly, sum of last 5 = 25 × 5 ⇒(b+c+d+e+f)=125 ----(ii) Subtracting equation (ii) from equation (i), ⇒ a−f=150−125=25 Thus, the differences of the first and the last numbers = 25