When I

When I am as old as my older brother is today, my brother will be twice as old as I am today.
a) How old is my brother if I am 8 years old today?
b) How old am I if my brother is 21 today?

Correct answer:

b =  12
k =  14

Step-by-step explanation:


j=8
j+x=b
b+x = 2·j

j = 8
b-j-x = 0
b-2j+x = 0

Pivot: Row 1 ↔ Row 2
b-j-x = 0
j = 8
b-2j+x = 0

Row 3 - Row 1 → Row 3
b-j-x = 0
j = 8
-j+2x = 0

Row 3 + Row 2 → Row 3
b-j-x = 0
j = 8
2x = 8


x = 8/2 = 4
j = 8/1 = 8
b = 0+j+x = 0+8+4 = 12

b = 12
j = 8
x = 4

Our linear equations calculator calculates it.

c=21
k+z=c
c+z = 2·k

c = 21
c-k-z = 0
c-2k+z = 0

Row 2 - Row 1 → Row 2
c = 21
-k-z = -21
c-2k+z = 0

Row 3 - Row 1 → Row 3
c = 21
-k-z = -21
-2k+z = -21

Pivot: Row 2 ↔ Row 3
c = 21
-2k+z = -21
-k-z = -21

Row 3 - -1/-2 · Row 2 → Row 3
c = 21
-2k+z = -21
-1.5z = -10.5


z = -10.5/-1.5 = 7
k = -21-z/-2 = -21-7/-2 = 14
c = 21/1 = 21

c = 21
k = 14
z = 7

Our linear equations calculator calculates it.



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