8+х+13+х=21+2х
3-в-7+в=-4
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0,5-2,7/8:(1/6+1,3/4)
1. 1/6+1,3/4=1,11/12
2. 2,7/8:1,11/12=1,1/2=1,5
3.0,5-1,5=-1