A dishonest dealer professes to sell his goods at the cost price but uses a false weight and thus gains 25%. How much quantity of grains does he give for a kilogram?

Correct option is B

Given:
The dealer gains 25% by using a false weight.
The dealer professes to sell 1 kg of grains.

Formula Used:
​$$\text{Profit}\% = \frac{\text{Selling Price} - \text{Cost Price}}{\text{Cost Price}} \times 100$$​​

Solution:

Selling Price = Rs. 1000 (for 1 kg)

Cost Price = Rs. Q (for Q grams).

​$$\text{Profit =} \frac{1000 - Q}{Q} \times 100 = 25 \\\ \\\quad \implies \quad \frac{1000 - Q}{Q} = 0.25 \\\ \\\quad \implies \quad 1000 - Q = 0.25Q \\\ \\\quad \implies \quad 1000 = 1.25Q \\\ \\\quad \implies \quad Q = \frac{1000}{1.25} = 800.$$​​

Thus, The correct answer is 800 grams