It takes the elf 80 seconds (1 minute and 20 seconds) to cut a sheet completely into strips, because 10 cm : strips of 2 cm=5 pieces and 4 cuts * 20 seconds each=80 seconds (the last cut makes two strips at once).
it would take an elf 80 second to cut the paper into strips as there are 5 strips per sheet and it requires 4 cuts to divide it into 5. 4 x 20 = 80.
however the question doesnt ask how long it takes for an elf to cut it. for me it would take about 15 seconds as I am much quicker than an elf