Answer:
5 (strawberries / hours)
Step-by-step explanation:
calculation fro morning 
strawberries / minutes  x   minutes / hours =   strawberries / hours 
  so after adding the value in above equation
3/4* 60/1  = 45 strawberries / hours
 calculation in the afternoon
strawberries / minutes  x   minutes / hours =   strawberries / hours
 2/3 x 60/1  =  40 strawberries / hours
  so now by calculating difference between morning and afternoon packing rates,  you can easily calculate 
 45-40 = 5 (strawberries / hours)