As per Indian Standard Soil Classification System (IS: $1498 – 1970$), an expression for $\textbf{A-line}$ is
1. $I_p=0.73(w_L-20)$
2. $I_p=0.70(w_L-20)$
3. $I_p=0.73(w_L-10)$
4. $I_p=0.70(w_L-10)$