If rahul sold the watch to sohan at gain of 10% =10/100 multiply by990=99
if sohan sold it at loss of 10%=99
if mohan paid 990 and we have to find the money paid by rahul =990-99=891
so rahul paid rs 891 for the watch
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Let the cost of watch which Rahul paid =x Rs
Rahul sold it at 10% profit =11x/10 Rs
Sohan sold it at 10% loss= 11x*9/10=99x/100
Mohan paid Rs 990
ATQ      99x/100=990
      ⇒    x=1000
Rahul paid Rs 1000
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