The weight of anything is determined on how much gravity is pulling on the object. For example a balloon full of helium floats away but iron or concrete would fall to the ground. hope this helps :)
Answer:
1 m = 100 cm....so 2.5 m = (2.5 * 100) = 250 cm
a = 1st shelf
b = 2nd
c = 3rd
d = 4th
a + b + c + d = 250
b = 2a + 18
c = a - 12
d = a + 4
a + (2a + 18) + (a - 12) + (a + 4) = 250
5a + 10 = 250
5a = 250 - 10
5a = 240
a = 240/5
a = 48 cm <== 1st shelf
b = 2a + 18 = 2(48) + 18 = 114 cm <== 2nd shelf
c = a - 12 = 48 - 12 = 36 cm <== 3rd shelf
d = a + 4 = 48 + 4 = 52 cm <== 4th shelf
so 2nd shelf is 114 cm
