HardMath and Logic
Measure 9 mins using 4-min and 7-min hourglasses.
Riddle
Measure 9 mins using 4-min and 7-min hourglasses.
Choose an answer
A.Flip the 7-min twice
B.Break the glass
C.Estimate with 4-min
D.Flip 4 at 4m, 7 at 7m, 7 at 8m
Hint: It's a 7-word phrase with 13 letters, starting with "F".
Answer
Flip 4 at 4m, 7 at 7m, 7 at 8m
Why this is the answer
Run both. When 4 ends, 3 mins are left in 7. Flip 4. When 7 ends (7m total), 1 min is left in 4. Flip 7. When 4 ends (8m total), 1 min has run into 7. Flip 7 back to get that 1m.
This riddle is filed under Expert Riddles for Adults and rated Hard.
Related Riddles
HardAdults & Lateral-Thinking
It takes a good scholar to learn what I am. I am full of knowledge and have many words.
HardAdults & Lateral-Thinking
In the first stages of my life, I am taller than I will be in my last stages.
HardAdults & Lateral-Thinking