Jack sold a jacket for 1800/=. He made a loss of 10%how much had he bought the jacket

Step 1: Determine the percentage of the cost price that the selling price represents. A loss of $10\%$ means that the selling price is $100\% - 10\% = 90\%$ of the original cost price.

Step 2: Set up an equation using the given information. Let the cost price be $C$. The selling price (SP) is $1800/=$. So, $90\%$ of $C$ is $1800/=$. $0.90 \times C = 1800$

Step 3: Solve for the cost price $C$. $C = \frac{1800}{0.90}$ $C = \frac{1800}{\frac{9}{10}}$ $C = 1800 \times \frac{10}{9}$ $C = \frac{18000}{9}$ $C = 2000$ Jack had bought the jacket for $\boxed{2000/=}$.