Let's calculate the costs step by step.

1. Cost for 20 notebooks and 11 desktop computers:

• Cost of 20 notebooks:
  $20 \times 150 = 3000$ dollars

• Cost of 11 desktop computers:
  $11 \times 200 = 2200$ dollars

• Total cost:
  $3000 + 2200 = 5200$ dollars

So, the total cost to buy 20 notebooks and 11 desktop computers is $5200.

2. Cost for nn notebooks and dd desktop computers:

• Cost of nn notebooks:
  $150 \times n$ dollars

• Cost of dd desktop computers:
  $200 \times d$ dollars

• Total cost:
  $150n + 200d$ dollars

So, the total cost to buy $n$ notebooks and $d$ desktop computers is $150n + 200d$ dollars.

Would you like more details or have any questions?

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Here are some related questions:

1. How many notebooks can be bought with a budget of $3000?
2. If the school decides to buy only desktop computers, how many can they purchase with $5200?
3. What would be the total cost if the school bought 15 notebooks and 8 desktop computers?
4. How much would it cost to buy 25 notebooks and 15 desktop computers?
5. If the school wants to spend exactly $6000, how many notebooks and desktops can they purchase in different combinations?

Tip: When dealing with budget problems, always break down the costs of individual items and calculate the total systematically to avoid mistakes.