A sample of an element X contains two isotopes X-35 and X-37 , if average atomic mass is 35.5u , calculate percentage of two isotopes in this sample.

Let 'x' be the % of mass number with 35.....and '100 - x' be the % of mass number with 37.

Atomic Mass = mass number 1 of X * %no. / 100 + mass number 2 of X * % no. / 100.

35.5 u = 35 * x / 100 + 37 * (100-x) / 100
35.5 u = 35x / 100 + 3700 - 37x / 100     (...remove the brackets)
35.5 u = 35x - 37x + 3700 / 100 (...calculation done under the common denominator 100)
35.5 * 100 = -2x + 3700 (...Balancing the equation)
3550 = -2x + 3700 
3550 - 3700 = -2x (...Balancing the equation)
-150 = -2x 
150 = 2x (...negative signs get cancelled on both LHS and RHS)
150 / 2 = x = 75%

Hence,  (100 - x) = 100 -75 = 25%

Thus, the % of element X with mass number 35 is 75% and % of same element X with mass number 37 is 25% !