Advanced Management Accounting

You can not have Slack for an Event
Slack is only for an Activity

okay then how to calculate slack of an activity?

L - E of Head Event to be considered in Free Float
L-E of Tail Event in case of Independent Float

But how to calculate then that's my query.
I didn't ask you about free float and independent float

Do you know how to calculate L and E?

Yes I know that. Just wanted to know how to calculate slack of head and tail events.
And even now you agreed that slack is for an event and not for activity.
Is it the difference between L and E?

I mean Slack you calculate for events but if you go conceptually it is for activity

Slack is the difference (irrespective of negative/positive difference) between E and L at any node. Considering an activity, head event slack is a slack at the end of the activity (slack at the node where the arrow ends) and tail event slack is the opposite (slack at the node where the arrow starts).
You may also note that slack at all nodes of a critical path wud always be zero.