Step-by-step explanation: If 5 elves can make 5 dolls in 5 minutes then an elf can make 5 dolls in twice the time it would take 5 elves to do it which is 25 minutes.
This means it takes an elf 25/5 = 5 minutes to make one doll
One elf can make 100 dolls in 100 x 5 = 500 minutes
Then it will take 100 elves 500/100 = 5 minutes to make 100 dolls.
NOTE: This method of solving is known as the unitary method. Be careful to note that it won't take 100 elves a hundred minutes to make 100 dolls because it's a concerted effort (each of the 100 elves is making 1 doll in the same time duration (5 minutes))
