Marianne is 20 - 8 = <<20-8=12>>12 years older than Bella.

Bella will turn 18 in 18 - 8 = <<18-8=10>>10 years.

So, Marianne will be 20 + 10 = <<20+10=30>>30 years old then.

# Answer

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Question:
Marianne was 20 years old when Bella was 8 years old. How old will Marianne be when Bella turns 18?
Answer:
Let's think step by step.

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30
Marianne was 20 years old when Bella was 8 years old. How old will Marianne be when Bella turns 18?
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