Answer:
David's orchid: 20 +x
Pam's orchid: 25 + 0.75x
Explanation:
Framing linear equations:
 David's apple orchid:
 Cost of first 10 pounds = 10 * 2 = $ 20
Let the weight of additional apples picked = x pounds
Cost of 'x' pounds = 1 * x = $ x
 
Total cost = cost of first 10 pounds + cost of additional 'x' pounds
 = 20 + x
Pam's orchid:
 Entry fee = $ 10
Cost of first 10 pounds = 10 * 1.50 = $ 15
 Cost of 'x' pounds = 0.75*x = 0.75x
Total cost = Entry fee + cost of first 10 pounds + cost of additional 'x' pounds
 = 10 + 15 + 0.75x
 = 25 + 0.75x