when u have chargable income of more than $20k, you are taxed.
e.g. Gross salary is $30k (including CPF)
Reliefs:
Provident Fund (CPF) - $6k (20% of gross salary)
Earned Income - $1k
Charageble income = $30k-$6k-$1k = $23k
Hence amount taxable will be $23k-$20k = $3k
Tax amount = $3,000*3.75% = $112.5