Q.A demand schedule shows that when price is Rs. 10, quantity demanded is 20 units; when price falls to Rs. 8, quantity demanded rises to 30 units. Treating price (P) as the Y-axis variable and quantity (Q) as the X-axis variable, find the slope of this demand line.

Slope measures how much price (the Y-axis variable) changes for each one-unit change in quantity (the X-axis variable), using the two given price-quantity pairs.

Step 1 — Identify the two points, with P on the Y-axis and Q on the X-axis: $(Q_1, P_1) = (20, 10)$ and $(Q_2, P_2) = (30, 8)$.

Step 2 — Apply the slope formula:

Step 3 — Independent check (dual-solve). Reversing the order of the two points must give the same slope: $m = \dfrac{10-8}{20-30} = \dfrac{2}{-10} = -0.2$. ✓ Matches.

Step 4 — Interpret. The slope is negative, as expected for a demand line — price and quantity demanded move in opposite directions. Its size, 0.2, means price falls by Rs. 0.2 for every one-unit rise in quantity demanded (equivalently, quantity demanded rises by 5 units for every Rs. 1 fall in price, since $1 \div 0.2 = 5$).

Swapping which variable is 'y' and which is 'x' (the axis convention in Section 3 puts P on the Y-axis, so the slope here is ΔP/ΔQ, not ΔQ/ΔP); dropping the negative sign, which is the economically meaningful part of the answer.