If it would take one machine 20 minutes to fill a large production order and another machine 30 minutes to fill the same order, how many minutes would it take both machines working together, at their respective rates, to fill the order?  Options:
A) 10 min
B) 12 min
C) 15 min
D) 20 min
E) 18 min

Rationale: this question can be solved using the work formula : work = ( a ) ( b ) / ( a + b ) where a and b are the individual completion rates of the respective entities ( for the same job ) . we ' re told that one machine takes 10 minutes to complete a job and another machine takes 12 minutes to complete the same job . plugging in those values gets us . . . ( 20 ) ( 30 ) / ( 20 + 30 ) = 600 / 50 = 12 minutes answer is b
Answer: B