Electronic device and method for mounting unit

A method for use with an electronic device that includes

• a frame having an upper row on which a first unit is mountable, and a lower row on which a second unit is mountable,
• a rack configured to accommodate the frame,
• a back plate configured to close the rack, and
• a duct configured to, while the first unit is mounted on the upper row and the second unit is mounted on the lower row, separate the first unit and the second unit, and protrude from an insertion hole formed in the back plate and thereby form an opening with a hole edge of the insertion hole, the method comprising:
• pushing the duct by pushing the second unit on to the lower row to mount the second unit on the lower row, thereby extending a protrusion length of the duct from the insertion hole and thereby increasing a cross-sectional area of the opening, and
• discharging, through the opening having the increased cross-sectional area, air passed through the second unit mounted on the lower row.