Question 420162
Let the bill amount be $x
Plan A 
70 + 0.4(x-70)
Plan B
130 + 0.2(x-130)
Plan B < Plan A
130+0.2(x-130)<70+0.4(x-70)
130+0.2x-26<70+0.4x-28
0.2x>130-70-26+28
0.2x>62
/0.2
x>310
The bill amount should be greater than $310