Trample, Assigning Damage, Multiple Blockers

[Missingkirby34]

This is probably very obvious but... If more than one creature say two 5/5 block a 15/15 with trample, and I choose to deal all 15 to one 5/5 does the opponent take 10?

[Keyeto]

No, lethal damage needs to be assigned to all creatures blocking it. From the Comprehensive Rules:
702.18b The controller of an attacking creature with trample first assigns damage to the creature(s) blocking it. Once all those blocking creatures are assigned lethal damage, any remaining damage is assigned as its controller chooses among those blocking creatures and the player or planeswalker the creature is attacking. When checking for assigned lethal damage, take into account damage already marked on the creature and damage from other creatures that's being assigned during the same combat damage step, but not any abilities or effects that might change the amount of damage that's actually dealt. The attacking creature's controller need not assign lethal damage to all those blocking creatures but in that case can't assign any damage to the player or planeswalker it's attacking.

RESOLVED

[Wally]

In short, if you only want to deal damage to one of those 5/5's (perhaps to avoid setting off a nasty dying trigger that the other might have), then you forfeit any more damage assigned by that attacking creature, trample or otherwise. Therefore 1 of the 5/5's has taken 15 damage and none was dealt anywhere else.