maths question 2

Q: Sally brown has an equal number of brothers and sisters. Mark

Brown, her older bother, has twice as many sisters as brothers.

How many children are there in the Brown family?

Explain your anser mathematically.

Sol:

Suppose there are x girls and y boys in the Brown family.

for Sally, she has x － 1 sisters and y brothers

x － 1 ＝ y ....... ( 1 )

for Mark, he has x sisters and y － 1 brothers

x ＝ 2 * ( y － 1 ) ....... ( 2 )

substitute y ＝ x － 1 into ( 2 )

x ＝ 2 * ( x － 1 － 1 )

x ＝ 2x － 4

0 ＝ x － 4

x ＝ 4

y ＝ x － 1 ＝ 4 － 1 ＝ 3

x ＋ y ＝ 4 ＋ 3 ＝ 7

Ans: There are 7 children in the Brown family.

Let x be the no. of Sally's sister
y be the no. of Sally's brother

x=y---------1
2(x+1)=y-1-------------2

Put 1 into 2,
2(x+1)=x-1
x=1

Put x=1 into 1
y=1