Jacks plane will normally cover 52 km in 12 min in still air. on one trip, he flew

  60 kph

Step-by-step explanation:

The relation between time, speed, and distance is ...

  time = distance/speed

  speed = distance/time

  distance = speed × time

___

Jack's plane flies at a speed of ...

  (52 km)/(0.2 h) = 260 km/h

If the wind speed is represented by w in km/h, then the distance remaining (in km) is ...

  (distance there) - (distance covered in 3.5 h) = 100 km

  (260 +w)(2.5) - (260 -w)(3.5) = 100

  -260 +6w = 100

  6w = 360

  w = 60

The wind speed was 60 km/h.

Check

The distance going was ...

  ((260 +60) km/h)(2.5 h) = 800 km

The distance covered in 3.5 hours coming back was ...

  ((260 -60) km/h)(3.5 h) = 700 km . . . . . 100 km short of the destination