coins problem

Taiwanpolitics SG

suppose u have 5 coins , 1 is fake , all the others r real . fake is lighter .

u nid to weight how many times to find the fake coin .

u r given a balance scale to weight .

second question , similar to first 1 but u dunno if the fake coin is havier or lighter than the real coin .

how many times u nid .

me noob in maths .

but my answer is 2 and 3 respectively .

me dunno correct or wrong .

Fatum SG

twice ...

once if you're lucky ....

Taiwanpolitics SG

firstly , weight 4 coins , 2 on each sides .

if scale is balanced , it means the xtra coin is fake .

if not balanced , it means the xtra coin is real .

then i c which side will tilt down , means heavier .

then i take the 2 coins from the other side ( lighter ) and measure with each other .

the heavier is real and lighter is fake .

freedom4ever SG

weight once will do.

Taiwanpolitics SG

if dunno which coin is lighter , fake or real .

same , place 2 coins on each side of scale .

if balanced means the xtra 1 is fake .

if unbalanced means xtra 1 is real .

the copy down will side is heavier and light .

then take 1 coin from the heavier side and compare with the real coin .

c if balanced , means that coin is real .
if not balanced , means the coin is fake .

c the fake coin heavy or light ........................

then use the above method , should get the answer 3 .

ami right to the second part ?

DriftingGuy SG

For 2nd question

1) Measure 2 coins per side, lets call them a & b vs c & d. If balanced out, the 5th coin is fake. If not go to step 2.

2) Measure a&b. If a&b balance out go step 3. If a&b not balanced go step 4.

3) Replace coin C with 5th coin. Measure 5th coin with D. If they balance, C is fake. If not balance, D is fake. Because at this point either C or D is fake. The 5th coin confirm is not fake.

4) Replace coin a with 5th coin. Measure 5th coin with b. If they balance, a is fake. If not balance, b is fake. Because at this point either a or b is fake. The 5th coin confirm is not fake.

All in all 3 measurements needed.

DriftingGuy SG

Ya its correct, you dont need to know whether the fake coin is heavier or lighter

mystiv SG

hmm... Laughing

freedom4ever SG

i guess i should explain my method. similar to yours at the first part.
first, i place the 2 coins. 1 at each side. if balance, continue to place another 2 coins. same thing, 1 at each side. if balance, place the last 2 coins, 1 at each side. in this way, i only get to weight 1 time.

ghimpheng SG

i just have to scalp off some metalic particles from the real coin and the real coin will be lighter that the other real coin that was not "handled" before

and balance it the real coin after scalping off some metalic particles will be surely lighter

Taiwanpolitics SG

ok

thanks for answering .

my maths sux .

can once meh .

this sure nid at least twice .

ItchyArmpit SG

many methods....... lucky can be 1
unlucky can be 2 or 3

other methods can be up to 4 ^^ Smile