If  (1+x)n=C0+C1x+C2x2+....+Cnxn then the value of ∑r=0n∑s=0n∑t=0nCrCsCt  is equal to

∑r=0n∑s=0n∑t=0nCrCsCt=∑r=0n∑s=0nCrCs∑t=0nCt =∑r=0n∑s=0nCrCs2n =∑r=0n2n Cr∑s=0nCs=∑r=0n2nCr2n=∑r=0n22nCr=22n∑r=0nCr=22n2n  = 23n