To calculate the monthly cost for Liam, we break the problem into two parts:

1. Fixed membership fee: $20 per month.
2. Cost per slice of pizza for members: $1.50 per slice.

Monthly cost for 11 slices:

$\text{Total cost} = \text{Membership fee} + (\text{Cost per slice} \times \text{Number of slices})$

$\text{Total cost} = 20 + (1.50 \times 11) = 20 + 16.50 = 36.50$

Monthly cost with 11 slices: $36.50

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Monthly cost for xx slices:

For xx slices, the formula remains the same:

$\text{Total cost} = 20 + (1.50 \times \text{xx})$

Monthly cost with xx slices: $20 + 1.50 \times \text{xx}$

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Let me know if you'd like more examples or further explanations! Here are some related questions for practice:

1. How much would Liam spend if he bought 20 slices in a month?
2. What is the breakeven number of slices where the membership becomes cost-effective compared to $2.50 per slice for non-members?
3. How much would Liam save per slice compared to non-members if he eats 11 slices?
4. If the membership fee increased to $25, how would it affect Liam's monthly cost for 11 slices?
5. What is the percentage discount per slice for members compared to $2.50 per slice for non-members?

Tip: Always consider fixed costs and variable costs separately to simplify budgeting problems!