A very simple maths answer that suddenly just couldn't come to my mind. Hope to see some guidance.

[Joanna_45]

If the hair treatment cost $160, mum gave $160. But daughter plan on returning the $100, so ended up the daughter actually spent how much on the hair treatment? ?

 

[BigBeachBanana]

The amount she spent on her hair and the amount she planned on returning to her mother must equal $160 (what her mother gave her in the beginning).
Let x be the amount she spent on her hair. Can you set this up as an equation?

 

[Steven G]

Think about where the money went. Remember think!
mum gave you $160 which you spent on the hair treatment. So far you spent nothing of your own money. Then you give your mum $100. How much do you think that you spent on this hair treatment?

 

[Steven G]

The amount she spent on her hair and the amount she planned on returning to her mother must equal $160 (what her mother gave her in the beginning).
Let x be the amount she spent on her hair. Can you set this up as an equation?

This is listed under pre-algebra so no variables.