A merchant claims a loss of 4% on coffee, but uses weight equal to 840g instead of 1000g. What profit/loss the merchant actually makes? A merchant claims a loss of 4% on coffee, but uses weight equal to 840g instead of 1000g. What profit/loss the merchant actually makes?

Merchant is giving goods worth Rs 840g for Rs 1000. So he is making a profit of $1000-840/840=400/21$.
Now given he makes a loss. Applying successive percentage 
$400/21-4-1600/21=1248/21$
That is loss. But my answer is not right
Suppose the cost price of coffee is 1 Rupee per gram. 

When he sells 1000 grams, he claims to make a loss of 4%. So, he claims that he is selling 1000 Rs goods at 960 Rs. 

He is actually selling 840 grams. So, his actual cost price is 840 Rs.

Gain = 960 - 840 = Rs. 120 

Profit % = (120/840)*100 = 14.28% 

You can check out problem 2 here, it also deals with cheating merchants / faulty weights.
Suppose the cost price of coffee is 1 Rupee per gram. 

When he sells 1000 grams, he claims to make a loss of 4%. So, he claims that he is selling 1000 Rs goods at 960 Rs. 

He is actually selling 840 grams. So, his actual cost price is 840 Rs.

Gain = 960 - 840 = Rs. 120 

Profit % = (120/840)*100 = 14.28% 

You can check out problem 2 here, it also deals with cheating merchants / faulty weights.