A trader purchases a watch and a wall clock for ₹ 390. He sells them making a profit of 10% on the watch and 15% on the wall clock. He earnes a profit of ₹ 51.50. The difference between the original prices of the wall clock and the watch is equal to

Let cost price of a wall clock = ₹ N
∴ Cost price of a watch = ₹ (390 - N)
According to the question,
{(N x 10)/100} + {(390 - N) x 15}/100 = 51.50
⇒ 390 x 15 - 5N = 51.50 x 100
⇒ 5N = 5850 - 5150 = 700
∴ N = 700/5 = 140
∴ Required difference = (390 - N) - N
= 390 - (2 x 140)
= 390 - 280 = 110
= ₹ 110