Question

how to calculate the atomic mass of oxigen

Answer 1

Divide the percentages by 100% to obtain a decimal figure and multiply each of the 3 masses (16,17 &18) by their respective decimal. 

99.76% / 100% = 0.9976: 
O:16 = 16 x 0.9976 = 15.9616. 

0.037% / 100% = 0.0037: 
O:17 = 17 x 0.00037 = 0.00629, 

0.204% / 100% = 0.00204: 
O:18 = 18 x 0.00204 = 0.03672. 
.........................Total = 16.004