Let the undiscounted price (original marked price) of the shirt be denoted by P.
The shirt was initially bought by the seller at a 10% discount. This means the seller's cost price (CP) was 90% of the undiscounted price (P): $CP = P \times (1 - 0.10) = 0.90P$
The shirt was then sold to a friend at a 10% loss. The selling price (SP) is given as Rs. 729.00. The loss is calculated based on the cost price (CP). $SP = CP \times (1 - 0.10) = 0.90 \times CP$
Substitute the known SP:
$729 = 0.90 \times CP$
Solve for CP:
$CP = \frac{729}{0.90} = \frac{7290}{9} = 810$
The seller's cost price was Rs. 810.
Now, we use the cost price (CP) found and the relationship from the initial purchase ($CP = 0.90P$):
$810 = 0.90P$
Solve for the undiscounted price (P):
$P = \frac{810}{0.90} = \frac{8100}{9} = 900$
The undiscounted price of the shirt was Rs. 900.