make group of 30 30 and 20 coins

in first weight 30 30

if they are same then coin is in group of 20 if not then it is in lighter one

after one weight u got 30 coins in which lighter is present

in second make three group of 10 10 10 and weight any two group

if both are same then coin is in third one else in a lighter one

after two weight u find 10 coins in which lighter is present

in third

make group of 3 3 4

and weight group of 3 3

if they are differ then u get the three else u get the group of 4

in fourth

if u get three weight any two and find the lighter one

but if u get four there is a need for fifth weight