Problem: The cost of 12 by 35 kg of mangoes is Rs 330 by 35. What is the cost of 12 by 5 kg of mangoes?

Explanation:
To solve this problem, we need to use the concept of proportionality. The cost of mangoes is directly proportional to the weight of mangoes. That means if the weight of mangoes increases, the cost also increases proportionally, and if the weight decreases, the cost also decreases proportionally.

Step 1: Find the cost of 1 kg mangoes.
We can find the cost of 1 kg mangoes by dividing the total cost by the total weight of mangoes.
Cost of 35 kg of mangoes = Rs 330
Cost of 1 kg of mangoes = 330/35
Cost of 1 kg of mangoes = Rs 9.43

Step 2: Find the cost of 5 kg mangoes.
We can find the cost of 5 kg mangoes by multiplying the cost of 1 kg mangoes by 5.
Cost of 5 kg of mangoes = 9.43 x 5
Cost of 5 kg of mangoes = Rs 47.15

Therefore, the cost of 12 by 5 kg of mangoes is Rs 47.15.

Conclusion: The cost of 12 by 5 kg of mangoes is Rs 47.15. We can use the concept of proportionality to find the cost of mangoes of any weight if we know the cost of mangoes per unit weight.