math Q (help!)

Q1. Mary bought a dress with 25% discount. She paid $240 for it.

(a) Find the original price of the dress. 這條是否這樣做: 240÷(1-25%)=$320

(b) If the cost of the dress is $100, find the percentage after selling the dress to Mary. 這條是否這樣做: [(240-100)÷100]×100%=140%
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Q2. A shop offers a 25% discount to a products A with marked price $800. The
profit of selling the products is $120.

(a) Find the cost of the products A.

(b) If the selling proce of another products B is $700 which makes a profit of
75%, is the cost of B higher than A? Explain your answer.
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Q3. 5 students finished a job in 6 days. For the same job, ｍ students finish
the job in ｎ days. (Assume that the efficiency of each student is the same.)

(a) Express ｍ in terms of ｎ. 這條是否這樣做: (5÷6) = (ｍ÷ｎ)
ｎ = 6ｍ/5

(b) If 12 students work together, how many days do they need to finish the job?
這條是否這樣做: ｎ= (6×12)÷5=14.4 (15 days)
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Q4. The standard deviation of the marks of S6 students in the mid-term
Mathematics examination is 8 marks. After reviewing the marked papers by the
teacher, he finds that there is a mistake of one question, thus 4 marks are
added to all students. Also, it is given that the weight of the mid-term
examination is 40%.

(a) Find the weighted standard deviation of the marks after adjustment.

(b) Will the standard score of each student be changed after adjustment?
Explain your answer.
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Thank you very much!