Question 286799
Another approach is 
6.5/10+6.5/x=1
6.5/x=3.5/10
65=3.5x
650=35x
650/35=x
65*5*2/5*7=x
65*2/7=x
130/7=x
x=18 4/7 hours
the first pump does 65% of the job and the second does 35%