How do I get the last coin at the World 5 Castle?

Get very small (or not, but my solution is esier) get to the block underneath the hole in the celinng and get up there mate!go in the coin direction and drop in there without dying

Here you go:

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| Level 5-Castle |
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Head right across the conveyor belt and get the Power-Up, followed up by
bypassing a Dry Bones and the Piranha Plants. When the conveyor belt
leads down to a ledge and a Piranha Plant is attacking from above, go low to
get the Star Coin, followed by leaping away to the right to escape the
carnivorous plant, leading Mario to face a series of Dry Bones and gaps,
arriving at a Power-Up. A Venus Fire Trap appears here, spitting fireballs
while Mario goes by, finding a series of Dry Bones to jump over while
staying stationary just to the right and below the ledge where the second
Star Coin twinkles, waiting for a springboard to come down the conveyor
belt, which is used to launch Mario to said Star Coin.

Next up are a Piranha Plant to pass to get to the halfway point, followed by
more Piranha Plant fun and a rotating Power-Up block (get the Mini Mushroom
if you intend to take the Alternate Route), leading into a meeting with a
Super Piranha Plant (let it fall to the floor below, then jump across the gap
to the conveyor belt above it's head). Next up is a ? Block to hit, so after
doing that, take a running leap from the right to land on the block, allowing
Mario to leap into the small opening above, Wall Jumping to get up onto the
ceiling, heading right over everything to find a Power-Up and the third Star
Coin. Now head through the large red doors above, entering into the Boss
Fight!