The fee for a transaction can be calculated by multiplying the number of operations the transaction contains with the base fee, which is 100 Stroops (0.00001 XLM).
Transaction Fee = (# of operations x base fee)
For example, a transaction that allows trust on an account’s trustline (operation 1) and sends a payment to that account (operation 2) would have a fee = 2 x 100 Stroops = 200 Stroops or 0.00002 XLM.
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