Free I.Q. Test: What is the minimum number of weighing operations

[MikeLigalig.com]

There are several versions of counterfeit coin puzzles where it is necessary to find the counterfeit coin, or coins, from a number of weighings. Most of these puzzles assume you have balance type scales available with two pans, where one object can be weighed against another. In this puzzle you have a single scale only with just one pan, which will weigh just one object, or group of objects at a time. You have three bags with an unspecified number of coins in each bag. One of the bags consists entirely of counterfeit coins weighing 75 grams each; the genuine coins weigh 70 grams each.

Question: What is the minimum number of weighing operations you need to carry out before you can be certain of identifying the bag of counterfeit coins?

source: Philip Carter and Ken Russell

[bol-anon nga cebuano]

it's really hard to tell but if the weight of the counterfeit and genuine coins are given. take 1 coin from one of the bags an weigh it and see what's the weight. if the weight matches one of the coins then it's easier for you to identify which is genuine and which is counterfeit. it will take 1-3 weighing.

[Mulligan]

Minimum is 2.

Why?  There are 3 bags in which one of the bags consists "entirely" of counterfeit coins.  

Sampling 1:  Genuine     = 70  grams
Sampling 2:  Counterfeit > 70  g

Maximum is 3.

Sampling 1:  Genuine     = 70  grams
Sampling 2:  Genuine    =  70  g
Sampling 3:  Counterfeit   >  70  g

Aw 1 diay.