Question 1042
Let the small number be x and the bigger one be y
:
x+y = 21
4x = 3y
:
For this two equations, we can use substitution
:
y = 21 - x (change it into y-form)
3y = 4x (another y-form)
:
now we have to take y away so we have to have 3y on one equation and -3y on another equation to get 0y which is nothing (hard isn't it?)
:
-3y = -63 + 3x
3y  = 4x
:
Now take away the ys by adding the equations
:
0 = 7x - 63
63 = 7x
x = 9
:
So now we know x and we can solve for y
:
3y = 4x
3y = 4(9)
3y = 36
y = 12
:
So the answers are 9 and 12