There's two ways to do this.
The first way:
Work out the amount of sugar needed to make one cake, and then multiply that by 7.
30 grams ÷ 4 cakes = 7.5 grams for 1 cake.
7.5 grams × 7 cakes = 52.5 grams for 7 cakes.
The second way:
Work out the ratio of sugar:cake by doing 7 ÷ 4, and then multiplying that value by 30.
7 cakes ÷ 4 cakes = 1.75
1.75 × 30 grams = 52.5 grams for 7 cakes.
Either way, the answer is 52.5 grams.