All times are in seconds in the precedence diagram.

In this precedence diagram the total task time is $135sec.$

Cycle time is $50s\frac{ec}{?}$ unit

The assembly line is as follows:

$WS1-CA$

$WS2-BE$

$WS3-D$

$WS4-FG$

What is the idle time for the assembly line listed above? $($in seconds$)$

$($Please note, you do not need to calculate TM$.)$