A shopkeeper mixes tea that costs ₹140 per kg with tea that costs ₹200 per kg, to make 30 kg of…

Step 1. The equations. The amounts: $x + y = 30$. The cost: $140x + 200y = 164 \times 30 = 4920$.

Step 2. Substitute. From the first equation $x = 30 - y$, so $140(30 - y) + 200y = 4920$. That is $4200 + 60y = 4920$.

Step 3. Solve. $60y = 720$, so $y = 12$, and $x = 18$.