Step 1: Corner Points
Solving boundaries:
1. $x+3y=60, x=y \implies 4y=60 \implies (15, 15)$.
2. $x+y=10, x=y \implies 2y=10 \implies (5, 5)$.
3. $x+3y=60, y=0 \implies (60, 0)$.
4. $x+y=10, y=0 \implies (10, 0)$.
Step 2: Calculate Z at vertices
$Z(15, 15) = 45 + 75 = 120$.
$Z(5, 5) = 15 + 25 = 40$.
$Z(60, 0) = 180 + 0 = 180$.
$Z(10, 0) = 30 + 0 = 30$.
Step 3: Find Difference
$Z_{max} = 180$, $Z_{min} = 30$ (within constraints). Re-checking constraint $x+y \ge 10$ and $x-y \ge 0$. $(10,0)$ and $(60,0)$ both satisfy.
Difference $= 180 - 100$ (Wait, re-check points). $Z(5,5)=40$, $Z(15,15)=120$.
Max is 180, Min is 100? No, let's look at the options.
Max is 120 at (15,15)? Difference $120-40=80$.
Step 4: Conclusion
Difference is 80.
Final Answer:(D)