i need help with some s.a.t questions!!!?

Question

1.) three tankers each carry 10,000 tons of oil and a fourth tanker carries 12,000 tons of oil.What is the average (arithmetic mean) number of tons of oil carried by each of the four tankers?

2.)The average (arithmetic mean) grade of ten students on a test is 64.What would an eleventh student have to score on the teat to bring the class average to 65?

3.)on a test 50 questions,Gertrude answered 47 correctly .If Gertrude answered every question, what percentage did she answer incorrectly?

4.) A merchant raises the price of a $100 item by 20 percent. After finding that heh cannot sell the item at the higher price, however, he discounts it by 20 percent. What is the final price of the item?

5.)if the ratio of boys to girls in a classroom is 1 to 3, what percent of the class is girls?

Answer 1

1) (3*10,000 + 12,000)/4 = 42,000/4 = 10,500

2) 64*10 + x = 65*11 --> x = 75

3) 47/50 = .94 = 94% right = 6% incorrect (sorry, misread)

4) 100*1.20 = 120; 120*(1-.20) = 120*.8 = $96

5) 75% (3 out of every 4) - sorry, I misread the question originally - 75% is the right answer

Answer 2

(10,000*3+12,000)/4 = 10,500 tons

(640+x)/11 = 65
640+x = 65*11 = 715
x= 715-640 = 775

3/50 = 6/100 = 6% incorrect answers

100*1.2 = $120 raised price
120 *.8 = $96 = final price.

For every boy there are three girls
So 3/4 of class are girls
3/4 = .75 = 75%

Answer 3

i agree with all of the answers except the number 5. there is 1 boy to every 3 girls, therefore, there are 75% girls in the class