A coconut vendor bought 6 dozen coconuts from a wholesale dealer at Rs 15 a dozen and sold them at the street at Rs 2 a piece. What was his profit%?
[a] 70%
[b] 60%
[c] 50%
[d] 40%

Hint: Find the total amount for which the seller bought the coconuts and the total amount for which the coconuts have been sold. Use the fact that the profit per cent of a transaction is the amount earned per 100 units of CP of that transaction. Hence determine the profit % of the transaction.

Complete step-by-step answer:
We shall first find the cost at which the coconuts were purchased.
Cost price of 1 dozen = Rs 15
Hence the cost of 6 dozen of coconuts = $15\times 6=90$
Hence the seller bought the coconuts for Rs 90.
Now, we know that 1 dozen of coconuts = 12 coconuts.
Hence the total number of coconuts purchased $=12\times 6=72$
Hence, we have CP = Rs. 90
Now, the cost at which 1 coconut is sold = Rs 2.
Hence the cost at which 72 coconuts were sold by the seller $=72\times 2=144$
Hence, we have SP = Rs 144
Hence the profit earned = Rs 144 -Rs 90 = Rs 54
Hence on the cost price of Rs 90 profit earned = Rs 54
Hence on the cost price of Re 1 profit earned $=\dfrac{54}{90}$
Hence on the cost price of Rs 100 profit earned $=\dfrac{54}{90}\times 100=60$
We know that profit % is the profit earned per 100 units of CP.
Hence, we have profit % = 60%.
Hence option [b] is correct.

Note: Alternatively, we can use the fact that
Gain % $=\dfrac{Gain}{CP}\times 100$
We can calculate the gain on 1 coconut and from that the gain%age of the transaction.
We have the cost of 12 coconuts = 15
Hence the cost of 1 coconut $=\dfrac{15}{12}=\dfrac{5}{4}$
CP of 1 coconut = $\dfrac{5}{4}$
Also, we have SP of 1 coconut = Rs 2.
Hence the amount gained on 1 coconut $=SP-CP=2-\dfrac{5}{4}=\dfrac{3}{4}$
Hence, we have
Gain % $=\dfrac{\dfrac{3}{4}}{\dfrac{5}{4}}\times 100=60$, which is the same as obtained above.
Hence option [b] is correct.