Chapter 5.5, Problem 15E

In Exercises 9-22, solve the given standard minimization problem using duality: (You may already have seen some of these in earlier sections, but now you will be solving them using a different method.] [HINT: See Example 1]

Minimize $c=2s+t$ subject to $3s+t\ge 30$

$s+t\ge 20$

$s+3t\ge 30$

$s\ge 0,t\ge 0$.