The ratio of the average of the marks obtained in Biology for two terms to the average of the marks obtained in English and Mathematics for first term only is:

$$\eqalign{ & \text{Average mark of biology} \cr & = \frac{90+55}{2} \cr & = \frac{145}{2} \cr} $$
Average of sum of Math first term & English first term
$$\eqalign{ & = \frac{100+70}{2} \cr & = \frac{170}{2} \cr & \therefore \text{Required ratio} \cr & = \frac{145}{2} : \frac{170}{2} \cr & = 29 : 34 \cr} $$