The half-life equation is given by:
\[
N = N_0 \times \left( \frac{1}{2} \right)^{\frac{t}{T}}
\]
Where:
- \( N_0 = 1 \, \text{gm} \) (initial mass)
- \( t = 600 \, \text{days} \)
- \( T = 150 \, \text{days} \) (half-life)
Substitute the values:
\[
N = 1 \times \left( \frac{1}{2} \right)^{\frac{600}{150}} = 1 \times \left( \frac{1}{2} \right)^4 = \frac{1}{16}
\]
Thus, after 600 days, the remaining mass is \( \frac{1}{16} \, \text{gm} \).