GMAT Quantitative Reasoning Practice Test 4

The length of all six snakes is $6\times 80=480\mathrm{cm}$.
Third of the snakes are by average $70\mathrm{cm}$ long therefore their sum is 140 .
The sum of the length for the remainder of the snakes is $480-140=340$. $\frac{340}{4}$ (snakes) is $85\mathrm{cm}$.