This is about simultaneous equations.
p = s has been proved below.
p + q = 25 - - - (eq 1)
r + s = 25 - - - (eq 2)
q = r - - - (eq 3)
p + r = 25 - - -(eq 4)
(p + r) - (r + s) = 25 - 25
p + r - r - s = 0
p - s = 0
p = s
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