Solve the following resource scheduling problem with the series method. The resource limit is $4$M$.$ What is the project duration after considering resource limits$?$

$($Hint: $\text{'}$M$\text{'}$ stands for Machinery, which is a type of resource. The limit is $4$M$,$ meaning that more than $4$ pieces of Machinery per day will not be available. The procedure to solve this is similar to what we already covered in class using the series method.$)$

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Group of answer choices

$16$

$6$

$18$

$20$