An Ideal Spring, with a Pointer Attached to Its End

An ideal spring, with a pointer attached to its end, hangs next to a scale. With a 100-N weight attached, the pointer indicates "40" on the scale as shown. Using a 200-N weight instead results in "60" on the scale. Using an unknown weight X instead results in "30" on the scale. The weight of X is:

A) 10 N
B) 20 N
C) 30 N
D) 40 N
E) 50 N