The easiest way to do this is to convert all the fractions into <em>minutes</em>
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So, she would've spent <u>1 hour and 30 minutes</u> on her homework and <u>40 minutes on her telephone</u>. If you <u>add</u> those amounts up, you get <u>2 hours and 10 minutes</u>. Since she goes to bed <u>3 hours after dinner</u>, and <u>2 hours and 10 minutes have gone by</u>, you do 3 hours - 2 hours 10 minutes and you get 50 minutes.
Hope that makes sense. Let me know if you have any questions.