Question 749509
First machine can do 1/10 mile per hour, Second machine, 1/18 mile per hour. 

1/10 + 1/18 = 9/90 + 5/90 = 14/90 = 7/45 miles per hour. 

A full mile would take 45/7 hours or ANS = 6 3/7 hours. 

Multiply by 20,  900/7 = ANS = 128 4/7 hours. 

Note - another tutor offered a short cut for this type of problem. 

A 10 hr worker and a 18 hour worker combined will do a job in (10*18)/(10+18) hours or 180/28 which is the same 45/7 hours I calculate. 
To be sure you get the numerator and denominator correct, be sure the answer makes sense. 2 10 hour workers would combine to do a job in 5 hours, so the 10 and 18 combine must be between 5 and 10.