The marks obtained by Alan in Mathematics, Eng lish and Biology are respectively 90 out of 100, 70 out of 150 and 150 out of 200. Find his average score in percent

(a) 87.83 (c) 76.33

(b) 68.88 (d) 77.33

The answer given is (90+70+150)/(100+150+200)=68.88%
Why can't the answer be (90/100+70/150+150/200)/3=70.05 %