The riddle was clever enough _______ even the brightest minds in the room.

• Enough (যথেষ্ট) শব্দটি adjective ও adverb হিসেবে ব্যবহৃত হয়। 
- Adverb/adjective এর পর 'enough' বসলে to-infinitive (to + verb এর base form) ব্যবহৃত হয়। 
Structure - adverb/adjective + enough + (to+verb+....) 
Example - He didn't run fast enough to win the race. 

আবার, adjective/adverb/verb + enough + for + pronoun (me/him/her, etc) + (to+verb) এরূপেও ব্যবহৃত হয়। 
Example - What he says is not clear enough for me to hear him clearly. 

• প্রদত্ত sentence এ 'clever' (adjective) এর পর 'enough' শব্দটি এসেছে। তাই to-infinitive (to + verb এর base form) ব্যবহৃত হবে। 

Complete Sentence - The riddle was clever enough to stump even the brightest minds in the room. 

Source: A Passage to the English Language by S.M. Zakir Hussain.