Explanation
Sean's distribution is reinvested at a NAVPU of $19.75 and he receives approximately 31.65 additional units.
When a mutual fund declares a distribution, it reduces its NAVPU by the amount of the distribution per unit.
In this case, the NAVPU drops from $21 to $19.75 after the distribution of $1.25 per unit. Sean's distribution is $625 ($1.25 x 500 units), which he reinvests in the mutual fund at the new NAVPU of $19.75. He receives

additional units. The total value of Sean's mutual fund holdings after the distribution and reinvestment is (500+31.65)×19.75=$10,500
, not $9,875. The NAVPU of the mutual fund does change after the distribution, regardless of whether Sean reinvests his distribution or not. References: [Unit 7: Mutual Funds Administration]