Answer:
1.46
Step-by-step explanation:
First find the the mean score before he made a 90.
[tex]\frac{75+78+83+80+79+84}{6} = 79.83[/tex]
Now add the score of 90 and calculate.
[tex]\frac{75+78+83+80+79+84+ 90}{7} = 81.29[/tex]
Subtract the two scores.
[tex]81.29-79.83 = 1.46[/tex]
His mean test score increased by 1.46.
