Question

To find her total earnings for those two weeks Nadia writes 7.5 x 18.5 + 20 = 158.75 explain her error

Answer 1

7.5 x 18.5 = 138.75 
138.75 + 20 = 158.75
Her error is not writing down her pay per hour then multiplying it by her hours per day, then multiply that by 14 to get the 2 weeks pay.
Hope this helped!