If your class is composed by [tex]n[/tex] students (you including), we deduce that [tex]n-1[/tex] students took the test. Let [tex]g_1,\g_2,\ldots,g_{n-1}[/tex] be the grades of your colleagues. We have
[tex]\dfrac{g_1+g_2+\ldots+g_{n-1}}{n-1}=69 \iff g_1+g_2+\ldots+g_{n-1}=69(n-1)[/tex]
If we consider your grade as well, the new average will be
[tex]\dfrac{g_1+g_2+\ldots+g_{n-1}+g_n}{n}=70 \iff \dfrac{69(n-1)+95}{n}=70[/tex]
Solving this equation for n, we have
[tex]\dfrac{69(n-1)+95}{n}=70 \iff 69n-69+95=70n \iff n=95-69=26[/tex]
