Let the cost price of the tea-set and the lemon-set be Rs x and Rs y respectively.
CaseI: When tea set is sold at 5% loss and lemon-set at 15% gain.
Loss in tea-set = Rs. 5x100= Rs. x20
Gain on lemon-set = Rs. 15y100= Rs. 3y20
∴ Net gain = Rs. 3y20−x20
⇒3y20−x20=7
⇒3y−x=140
⇒x−3y+140=0 (i)
CaseII: When tea-set is sold at 5% gain and the lemon-set at 10% gain.
Gain on tea-set = Rs. 5x100= Rs. x20
Gain on lemon-set = Rs. 10y100= Rs. y100
∴ Total gain = Rs. x20+y10
⇒x20+y10=13
⇒x+2y=260
⇒x+2y−260=0 .(ii)
Subtracting equation (ii) from equation (i), we get
−5y+400=0⇒y=80
Putting y=80 in equation (i), we get
x−240+140=0⇒x=100
Hence, the cost prices of tea-set and lemon-set are Rs. 100 and Rs. 80 respectively.