I provided the biweekly federal tax table for convenience. If this is not the updated federal tax table, then just understand the concept. The same procedure would be applicable.
On the lefthand-most column, you find the row for $840. Then, at the top row, it indicated the exemption. Since Selina has a single exemption, find the column with 1 in it. The intersection of the designated row and columnis Selina's withheld amount. That would be 80. We find the 21% of this.
80*0.21 = $16.8
Thus, Selina's amount of state tax will be $16.8.