Can you use "ti" as well as "poia" here? Does anyone know?
Ποιος/ποια/ποιο are used for both 'who' and 'which one'. Τι is used to literally ask 'What is this?' Saying "Τι είναι η ταχύτητα του αυτοκινήτου;" means 'What does η ταχύτητα του αυτοκινήτου mean?' So, no, you can't use τι here. :)
please, please, please, add a greek keyboard just as other languages in duolingo have already done. That would helps us a lot, thanks.
Τι είναι η ταχύτητα του αυτοκινήτου; Ναι ή όχι;
No, because "Τι + είναι + xxx" is like asking for the definition of "xxx".
Ευχαριστώ πάρα πολύ.