Telangana PSC Group 1 Prelims 2016 Paper

A vessel contains 100 coins of five different colours, each colour contributing 20 coins. Coins of four colours weigh one gram each. Coins of the remaining colour weigh 0.9 gram each. If we use a balance and suitable measuring weights, then what is the minimum number we need to find out the colour of the coins that weigh 0.9 gram?