Answer: Â 12 minutes
Step-by-step explanation:
Two rates for hoses A and B. Â FP is the Fish Pond.
A = 20 min/FP Â or (1/20)FP/min
B = 30 min/FP Â or (1/30)FP/min
The time in minutes, X,  to fill  one FP:
           1FP = X*((1/20)FP/min) + X*((1/30)FP/min) Â
           1FP = X*(1/20+1/30)FP/min
           1FP = X*(2.5/30)FP/min
           X = 12 minutes