You can do this problem algebraically by assigning variables. If Sara has 3 times Sam we can say Sam is x and Sara is 3x and together they have 28.
Which looks like
3x + x = 28
4x = 28 ( divide both side by 4)
x= 7
So Sara have 3x which after substitution or replacement looks like this
3 x 7 = 21.
So Sara have 21 pencil eraser.