Question

a merchant sold his goods for Rs75 at a profit% is equal to cost price find c.p

Answer 1

The answer is Rs.50 - Check this - when you add on 50% to Rs.50, you get Rs.75 

If you need to show working: 

C (1 + C/100) = 75 
C + C^2/100 = 75 
100C + C^2 = 7500 
C^2 + 100C - 7500 = 0 
(C - 50) (C + 150) = 0 
C = 50 (ignoring the negative solution)

Answer 2

ᴛʜᴇ ᴀɴsᴡᴇʀ ɪs ʀs.50 - ᴄʜᴇᴄᴋ ᴛʜɪs - ᴡʜᴇɴ ʏᴏᴜ ᴀᴅᴅ ᴏɴ 50% ᴛᴏ ʀs.50, ʏᴏᴜ ɢᴇᴛ ʀs.75 

ɪғ ʏᴏᴜ ɴᴇᴇᴅ ᴛᴏ sʜᴏᴡ ᴡᴏʀᴋɪɴɢ: 

ᴄ (1 + ᴄ/100) = 75 
ᴄ + ᴄ^2/100 = 75 
100ᴄ + ᴄ^2 = 7500 
ᴄ^2 + 100ᴄ - 7500 = 0 
(ᴄ - 50) (ᴄ + 150) = 0 
ᴄ = 50 (ɪɢɴᴏʀɪɴɢ ᴛʜᴇ ɴᴇɢᴀᴛɪᴠᴇ sᴏʟᴜᴛɪᴏɴ)