Given inequality is, 4x−16x<21⇒4x−112x<1 ⇒4x−112x−1<0⇒4x−18x+1<0 ...(i)
For above inequality (i), we have two cases either (I)8x+1>0,4x−1<0
or (II)8x+1<0,4x−1>0
From (I), we get x>8−1,x<41 ∴x∈(−81,41) ....(ii)
From (II), x<8−1,x>41 , both of which can not be true ∴ (ii) represent the solution set of given inequality.