$12\frac{6}{17}+\dots ..=15\frac{1}{17}$

$15\frac{1}{17}-12\frac{6}{17}=14\frac{18}{17}-12\frac{6}{17}=2\frac{12}{17}$

$12\frac{6}{17}+2\frac{12}{17}=15\frac{1}{17}$