1. Problem statement: We are given market equilibrium for pencil prices and quantities. At price Rp 1,000, demand equals supply at 800 pencils. When price rises to Rp 1,500, demand is 300 and supply is 1,800. We need to find: a. Demand function b. Supply function c. Excess at price Rp 1,200 and its amount 2. To find the demand function, assume a linear form $Q_d = a - bP$. At equilibrium price 1,000, $Q_d = 800$: \(800 = a - b(1000)\). At price 1,500, demand is 300: \(300 = a - b(1500)\). Subtracting equations to solve for $b$: $$800 - 300 = (a - 1000b) - (a - 1500b) = 500b \Rightarrow 500 = 500b \Rightarrow b = 1.$$ Substitute back to find $a$: $$800 = a - 1000(1) \Rightarrow a = 1800.$$ Therefore, demand function: $$ Q_d = 1800 - P. $$ 3. To find the supply function, assume linear: $Q_s = c + dP$. At equilibrium (price 1,000), supply is 800: $$ 800 = c + d(1000). $$ At price 1,500, supply is 1,800: $$1800 = c + d(1500).$$ Subtract to solve $d$: $$1800 - 800 = d(1500 - 1000) \Rightarrow 1000 = 500d \Rightarrow d = 2.$$ Substitute back for $c$: $$800 = c + 2 imes 1000 \Rightarrow c = 800 - 2000 = -1200.$$ Supply function: $$ Q_s = -1200 + 2P. $$ 4. To find excess at price 1,200: Calculate demand: $$ Q_d = 1800 - 1200 = 600. $$ Calculate supply: $$ Q_s = -1200 + 2(1200) = -1200 + 2400 = 1200. $$ Excess supply (since $Q_s > Q_d$) is: $$ Excess = Q_s - Q_d = 1200 - 600 = 600. $$ Final answers: a. Demand function: $$Q_d = 1800 - P$$ b. Supply function: $$Q_s = -1200 + 2P$$ c. Excess supply at price 1,200 is 600 units.