Math problem in grade six: the monthly fee for card A is per minute 15 yuan, and the monthly fee for card B is 0. 15 yuan. There is no 0.3 yuan per minute, and it will be charged according to the average minutes of calls per month. Assume that the average monthly call time is x minutes.

Total monthly fee for card A = 15+0. 15X.

Total monthly fee for card B = 0.3x.

Then the cost is the same.

15+0. 15X=0.3X

∴X= 100

That is, when the average monthly call is 100 minutes, the charge is the same.

When the B card is cheap, then

15+0. 15X>0.3X

∴X< 100

That is, when the average monthly call is less than 100 minutes, the B card is cheaper.