The value of limn→∞ ∑r=1n ∑t=0r−1 15n⋅nCr⋅rCt⋅3t is equal to
limn→∞ ∑r=1n 15n⋅nCr∑t=0r−1 rCt⋅3t=limn→∞ ∑r=1n 15n⋅nCr4r−3r=limn→∞ 15n∑r=1n nCr4r−∑r=1n nCr3r=limn→∞ 15n5n−4n=1