One hose can fill a goldfish pond in 24 minutes, and two hoses can fill the same pond in 15 minutes. find how long it takes the second hose alone to fill the pond.

2 hoses together:
15 mins = 1 pond
1 min = 1/15 pond

1st hose:
24 mins = 1 pond 
1 min = 1/24 pond

2nd hose:
1 min = 1/15 - 1/24 = 1/40 pond

So 1/40 of the pond = 1 min
   40/40 of the pond = 1 x 40 mins
          1 whole pond =  40 mins


it will take 40 mins for the 2nd hose to fill up the pond alone.