This question is essentially a "Work Formula" question, since it has 2 people working on a task together. You have to be really careful with the details though, since the two given rates are NOT for the same task....
We're told that Danny can finish off the jar of candies in 5 minutes
We're told that Ian can finish off HALF the jar of candies in 10 minutes, which means that he can finish the WHOLE jar in 20 minutes.
We're asked how long it would take the two of them to finish the jar of candies together....
Work = (A)(B)/(A+B) where A and B are the individual rates of the two entities.
A = 5
B = 20
(5)(20)/(5+20) = 100/25 = 4 minutes to finish off the jar of candies.
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