Q.In a market, the demand function for a commodity is $Q_d = 200 - 4P$ and the supply function is $Q_s = 20 + 6P$. Find the equilibrium price and equilibrium quantity. Also state what would happen if the price were fixed at ₹20.

Equilibrium price is found where quantity demanded exactly equals quantity supplied.

Step 1 — Set quantity demanded equal to quantity supplied.

Step 2 — Find the equilibrium quantity.

Using the demand function: $Q_d = 200 - 4(18) = 200 - 72 = 128$.

Step 3 — Verify with the supply function.

$Q_s = 20 + 6(18) = 20 + 108 = 128$. Both sides agree, confirming $P=₹18$ and $Q=128$ units is the equilibrium.

Step 4 — Effect of fixing price at ₹20 (above equilibrium).

$Q_d = 200 - 4(20) = 120$. $Q_s = 20 + 6(20) = 140$. …

Cross-check by testing a price below equilibrium, say ₹15. $Q_d = 200-4(15)=140$; $Q_s = 20+6(15)=110$. Since $Q_d(140) > Q_s(110)$, this confirms a shortage below ₹18 and a surplus above it — consiste …

Solving $Q_d = Q_s$ correctly for $P$ but forgetting to substitute back into EITHER function to report the equilibrium quantity as a separate, explicit number; also confusing 'surplus' (supply exceeds demand, at too HIGH a price) wit …