How do you pronounce challenge?

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”.

Is the G silent in length?

So, the “g” is not actually pronounced as a “g,” but it is not a “silent” letter either. It is actually pronounced in words such as “anger” and “linger”—but the same sort of thing does not happen in “strength” and “length” because of the “th” sound that follows immediately after.