Method A charges 0.1X

Method B charges 20+0.05X

Because 0.1X-(20+0.05X)=0.05X-20

When X=400 minutes, 0.05X-20=0. At this point, both methods charge the same amount

When X>400 minutes, 0.05X-20=0. At this point, Method B is cost-effective

When X＞400 minutes, 0.05X-20＞0, at this time, way B is cost-effective

When X＜400 minutes, 0.05X-20＜0, at this time, way A is cost-effective

Wish you happiness