Price elasticity of demand is (-1), at a given price the consumer buys 60 units of the good. How many units will the consumer buy if the price falls by 10%:

Step 1: Identify the given values. Price elasticity of demand (PED) is given as $-1$. Initial quantity demanded ($Q_1$) is 60 units. The price falls by $10\%$, so the percentage change in price ($\% \Delta P$) is $-10\%$.

Step 2: Use the price elasticity of demand formula to find the percentage change in quantity demanded. The formula for price elasticity of demand is: $PED = \frac{\% change in quantity demanded}{\% change in price}$ Substitute the given values into the formula: $-1 = \frac{\% change in quantity demanded}{-10\%}$ Now, solve for the percentage change in quantity demanded ($\% \Delta Q$): $\% \Delta Q = -1 \times (-10\%) = 10\%$

Step 3: Calculate the new quantity demanded. A $10\%$ increase in quantity demanded means the quantity will increase by $10\%$ of the initial quantity. Increase in quantity = $10\%$ of $60$ units $Increase in quantity = 0.10 \times 60 = 6 units$ The new quantity demanded ($Q_2$) is the initial quantity plus the increase: $Q_2 = Q_1 + Increase in quantity = 60 + 6 = 66 units$ The consumer will buy $\boxed{66 units}$.