Demand curve: Q = 1200 - 10p

Price: p = 5

DQ/dp: 10

Elasticity: ?

Calculating elasticity:

Formula: Elasticity (E) = (% Change in Quantity Demanded / % Change in Price)

Step 1: Calculate the current quantity demanded at price p = 5

Q = 1200 - 10p
Q = 1200 - 10(5)
Q = 1150

Step 2: Calculate the new quantity demanded when price changes by 1%

New price = p + (1% of p)
New price = 5 + (0.05)
New price = 5.05

New quantity demanded = 1200 - 10(5.05)
New quantity demanded = 1147.5

Step 3: Calculate the % change in quantity demanded

% Change in Quantity Demanded = (New Quantity Demanded - Original Quantity Demanded) / Original Quantity Demanded x 100%

% Change in Quantity Demanded = (1147.5 - 1150) / 1150 x 100%
% Change in Quantity Demanded = -0.217%

Step 4: Calculate the % change in price

% Change in Price = (New Price - Original Price) / Original Price x 100%

% Change in Price = (5.05 - 5) / 5 x 100%
% Change in Price = 1%

Step 5: Calculate the elasticity

Elasticity (E) = (% Change in Quantity Demanded / % Change in Price)

Elasticity (E) = (-0.217% / 1%)
Elasticity (E) = -0.217

Conclusion: The elasticity at Q=1200-10p when p=5 and DQ/dp = 10 is -0.217, which means that the demand is relatively inelastic. This means that a change in price will result in a proportionally smaller change in quantity demanded.