4L + 10S = 200
6L + 5S = 200.....multiply by -2
------------------
4L + 10S = 200
-12L - 10S = - 400 (result of multiplying by -2)
-----------------add
-8L = -200
L = -200/-8
L = 25
4L + 10S = 200
4(25) + 10S = 200
100 + 10S = 200
10S = 200 - 100
10S = 100
S = 100/10
S = 10
so the small plates (S) weigh 10 lbs and the large plates (L) weigh 25 lbs