Let
x--------> amount of erasers that Sari has
y--------> amount of erasers that Sam has
we know that
[tex] x=3y [/tex]-------> equation [tex] 1 [/tex]
[tex] x+y=28 [/tex]-----> equation [tex] 2 [/tex]
Substitute equation [tex] 1 [/tex] in equation [tex] 2 [/tex]
[tex] [3y]+y=28\\ 4y=28\\ y=28/4\\ y=7 [/tex]
find the value of x
[tex] x=3y\\ x=3*7\\ x=21 [/tex]
therefore
the answer is
Sari has [tex] 21 [/tex] erasers