Let's say there are S students. Let C be the number of students having a computer. Those not having a computer will be S-C. So the ratio is C/S:(S-C)/S=C:(S-C). If P is the number of students with a cellphone, then S-P is the number without and the ratio is, similarly, P:(S-P). To compare the two ratios we need actual figures. Then we compare ratios as we would fractions: C/(S-C)-P/(S-P)=(CS-CP-PS+CP)/((S-C)(S-P))=S(C-P)/((S-C)(S-P)). If this expression is positive C>P and the computer ratio is higher, otherwise the phone ratio is higher.
Example: Let S=100, C=85 and P=95. The comparison expression is 100*(-10)/75=-40/3. The ratios are 85:15=17:3 and 95:5=19:1. 17/3-19=-40/3. In this example the phone ratio is higher.