Call the plans A and B. Call the number of texts T.
A=0.10(T-500) and B=0.20(T-400). If A=B, 0.10T-50=0.20T-80.
30=0.10T, T=300, but this is less than the number required for free texts. A and B=-20.
The standard charge is required to make comparisons.
If a is the standard charge for A and b for B,
30+a-b=0.10T and T=10(30+a-b). The difference in the standard fixed charges impacts on T.
This means that T>500, 10(30+a-b)>500, 30+a-b>50, a-b>20. For example if a-b=50, T=800.