Suppose Zhang sends an average of m text messages every month, and the monthly phone bill is Y yuan.

Then the first activity: (1) m

(2)m & gt； 140, then y2=42+0. 1*(m- 140).

The second activity: Y3 = (0.3 *140+0.1* m) * 0.8.

If the second method is economical and effective, then (1) m

(2)m & gt； At 140, y2 >；; y3 .

Solve m

Therefore, when the number of short messages is less than 105 or more than 280, it is more cost-effective to choose the second method.