Question 897819
rate for Bob (A) alone 10
rate for A & B together 6

1/a+1/b=1/x
1/10+1/b=1/6
1/b=1/6-1/10
1/b=10/60-6/60
1/b=4/60
1/b=1/15
b=15
rate for Frank (B) alone 15