Sheila has five times as many markers as days together they have 18 marbles how many markers does sheila have

Dave has X markers, so Sheila has 5X markers.  Adding up all markers, X + 5X = 6X. There are 18 markers, so 6X=18.  Dividing both sides of that equation by 6, we get X=3. 5X is 15.   So Dave has 3 markers and Sheila has 15 markers, for a total of 18 markers

I love putting algebra into things so let me have a go

sheila will be S
im assuming days is the other person so they shall be D
so
S=5D as she has 5 times the amount that D has right.
and D and S together make 18
so
D+S=18
but as S is also 5D, lets substitute that into this equation we made
so it changes from
D+S=18
to
D+5D=18
we can collect these like terms and show it as
6D=18
we can now simplify this by dividing both sides by 6
D=18/6
D=3

were not done just yet :p
so we figured out how many DAYS has but notbyet shiela
we can find this by 2 ways
one way:
if both added together is 18, and we minus days amount, well be left with shielas amount
so 18-3=15
so shiela has 15
OR:
other way:
shiela has 5 times the amount as days
lets multiply days amount by 5 to get shielas
3×5=15

either way same answer

hope this helps X