Question 198519
X is the number of days he work
(30-X) is the number of days he doesn't work 
He will receive $12*X after X days of working
He will have to pay his employer $12*(30-X) for the days that he idle
At the end of 30 days he earns $240 means the money he earn subtract the money he pay his employer for idling days  is $240=$12*X-$12*(30-X)
$240=$12*X -$360 + $12*X
$600=$24*X
25=X
He work 25 days and idle 5 days