Compute the amount to be paid for each of the four separate invoices assuming that all invoices are paid within the discount period.

$\backslash$table$[[,\backslash$table$[[$Merchandise$],[($gross$)]],$Terms,Payment$],[$a$.,$$$,6,200,\frac{2}{10},\frac{n}{60},],[$b$.,$$$,21,800,\frac{1}{15},\frac{n}{90},],[$c$.,$$$,77,400,\frac{1}{10},\frac{n}{30},],[$d$.,$$$,13,000,\frac{3}{15},\frac{n}{45},]]$