What is a meaning of adamant?

: unshakable or insistent especially in maintaining a position or opinion : unyielding an adamant insistence on doing things his own way was adamant about making the change.

Is the T silent in exactly?

In practice, yes it is virtually silent unless one makes a special effort to pronounce it. When we have a consonant cluster here /ktl/ one sound is often not heard. The linguistice term for it is “dissimilation”.

How is 2020 pronounced?

You’d say: twenty twenty. The second is to say: two thousand and twenty.