F.1's math

Q.2 :

Let x be his sister's age after 5 years

15+5+x = 2x
20+x = 2x
20 = 2x-x
x = 20

Thus, his sister is (20-5) 15 years old
he is (20+15+5-5) 35 years old

2007-02-12 09:03:39 補充：
Q.3 :Let the number of questions he ans correctly be x5x-3(16-x) 0(20-16) = 565x-48 3x = 568x-48 = 568x = 116x = 13Ans: The number of questions he ans correctly are 13.

Q.2

Let his sister's age = s,
then, as he is 15 years older than his sister, thus his age = s + 15

5 years later, his sister's age = s + 5
5 years later, his age = s +15 + 5 = s +20

5 years later, he will be twice as old as his sister , thus
his age 5 years later = 2 x ( his sister's age 5 years later )
s + 20 = 2 x (s + 5)
s + 20 = 2s + 10
s + 20 - 10 = 2s +10 - 10
s +10 = 2s
s +10 - s = 2s - s
10 = s

therefore his sister's age now is s = 10 years old
his age now is s + 15 = 10 +15 = 25 years old


Q.3

let the no. of questions he answered correctly be c
then, 5 marks will be given for correct answer, the marks he got for correct answer = 5c

as he answered 16 questions only, thus, the no. of questions with wrong answer = 16 - c
then, 3 marks will be deducted for wrong answer, the marks he got for wrong answer = -3 x (16 -c)

& 4 questions are unanswered

total marks = marks for correct answer + marks for wrong answer + marks for no answer
56 = 5c -3 x (16 - c) + 4 x 0
56 = 5c - 48 + 3c + 0
56 + 48 = 8c -48 + 48
104 = 8c
c = 13

therefore, no. of questions he answered correctly = 13