First you add the ratio together 3+7=10 Then you do £300 divided by 10 = £30
Then you do £30 X 3 (from the ratio in the beginning) which equals £90
Then you do £30 X 7 (also from the ratio in the beginning) which equals £210
The ratio is £90:£210
Also you can add the ratios up so that it equals £300 the number you started with
£90+£210=£300
Hope I helped :)