Question

according to law of triads find the atomic mass of an element x, where y =80 and z =127.

Answer 1

In the year 1829,  a German chemist Dobereiner proposed law of Triads which states that " when group of three elements[called as triad] are arranged in increasing order of atomic masses, the atomic mass of middle element of triad will be approximately equal to the arithmetic mean of atomic masses of other two elements"
So according to triad :
X=?
Y=80
Z=127
so they form the triad from Halogen Group: Cl, Br,I
Cl  -- Atomic weight -35.5
Br  -- Atomic weight -80
Iodine - Atomic weight -127
so, 35.5+127/2 ≈81.2
Hence element X= Chlorine with atomic weight 35.5

Answer 2

x, y and z will be in order 
x+z/2 = y
x + 127/2 = 80
x = 160 - 127
x= 33

so the atomic mass of element x is equal to 33