Let x be the cost of the bike for A.
A sells it to B with a 28% loss
So B pays (1-28/100)x = 0.72x
B spends $1680, so he has now spent 0.72x+1680
B sells it to C making a 12.5% profit.
So B sells it to C for (1+12.5/100)(0.72x+1680) = 1.125(0.72x+1680) = 0.81x+1890
We are told C paid $35910,
So 0.81x+1890 = 35910
x = (35910 - 1890)/0.81 = 42000
So A paid $42000
(We could have also just worked backward. B sold the bike for 35910 and made 12.5% profit, so it originally cost him 35910/1.125 = 31920. B spent 1680 on repairs, so before repairs it cost him 31920-1680 = 30240. A sold it for 30240 which was a 28% loss, so it originally cost 30240/0.72 = 42000.)
Hope this helped,