Hello from MrBillDoesMath!
Answer:
L = 35, W = 25
Discussion:
The attachment shows that
P = 2L + 2W = 120
and
L = 10 + W
2L + 2W = 120 => Substitute the last equation into the first one
2(10 + W) + 2W = 120 =>
20 + 2W + 2W = 120 => combine like terms
20 + 4W = 120 => subtract 20 from both sides
4W = 120 -20 = 100 => divide both sides by 4
W = 100/4 = 25
Hence, L = 10 + W = 10 + 25 = 35
Thank you,
MrB