Were does the gain and reserve calculation's come from?

B sold capital property in the current year for net proceeds of $500,000. The property has an adjusted cost base of $100,000. B received $200,000 at the time of the sale and a note for the balance to be paid in equal annual instalments over the following three years. What is the minimum taxable capital gain to be reported in the current year? $ 62,500 Explanation [(Gain $400,000 - Reserve $240,000) * 1/2]