Student A, B and C con complete a puzzle in 20 min, 15 min, and 30 min respectively. If all three student work together, how long will it take to complete the puzzle?

Answer 1

use the same method I just showed you in your previous post.
Let me know what you got.

Answer 2

1/20 + 1/15 + 1/30 = 3/60 + 4/60 + 2/60 = 9/60
0.15 = 15 min
I think is wrong?

Answer 3

You followed my steps only up to a point

combined rate = 9/60 ---- correct

time at combined rate = 1/(9/60) = 1 ÷ (9/60)
= 1 (60/9)
= 60/9 = 15 minutes

You just divided 9 by 60 to get .15
and it is just coincidence that the digits of .15 and the correct answer of 15 are the same