Final answer:
Tom gained $12 by selling the pen at 20% profit on his original cost price of $60.
Step-by-step explanation:
Taking the problem statement, Tom bought a pen for $60 and sold it to gain 20% on his cost price, we can calculate his gain. To find the gain, we first need to calculate 20% of the cost price.
20% of $60 is calculated as (20/100) × $60 = $12.
This means Tom sold the pen for $60 (cost price) + $12 (gain) = $72. Therefore, the actual gain Tom made from the sale is $12, which corresponds to option (a).