Question 294484
1.{{{A1=(J+K+L+M)/4=75}}}
2.{{{A2=(J+K+L+M+N)/5=80}}}
where J,K,L,M are the grades from the first 4 tests, N is the next test grade.
From eq.1, 
{{{J+K+L+M=300}}}
Use that in eq.2, 
{{{J+K+L+M+N=400}}}
{{{300+N=400}}}
{{{N=100}}}
He needs to get 100 on his next test.