First multiply Marla's one-way driving time by 2 to find her total driving time: 20 minutes * 2 = <<20*2=40>>40 minutes

Then add Marla's driving time to the time spent attending the conference to find the total time: 70 minutes + 40 minutes = <<70+40=110>>110 minutes

# Answer

110
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Marla has to spend 20 minutes driving one way to her son's school, then 70 minutes attending parent-teacher night, then the same amount of time driving home. How many minutes does Marla spend on this errand in total?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
110
Marla has to spend 20 minutes driving one way to her son's school, then 70 minutes attending parent-teacher night, then the same amount of time driving home. How many minutes does Marla spend on this errand in total?
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