x/1×2 + x/2×3 + x/3×4 + ... + x/99×100 = 99x - x/2 + x/2 - x/3 + x/3 - x/4 + ... + x/99 - x/100 = 99x(1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + ... + 1/99 - 1/100) = 99x (1 - 1/100) = 9999x/100 = 99x = 100
x(1-1/100)=99x=100