Hey there!
Marked price (MP) = 1100 + Cost price (CP) ----(1)
If the watch was sold at discount of 20%, profit = Rs. 400
⇒ Selling price = MP - 20% of MP = 0.8MP
Also, Selling price = CP + 400
⇒ CP + 400 = 0.8MP
Substituting from (1),
⇒ CP + 400 = 0.8(1100 + CP)
⇒ CP + 400 = 880 + 0.8CP
⇒ 0.2CP = 480
⇒ CP = Rs. 2400
∴ To earn a profit of 30%, Selling price = 2400 + 30% of 2400 = 2400 + 720 = Rs. 3120
