Skirts are common in both ratios.

Multiply both ratios by the factors such that skirts are represented by the same number

Dresses: skirts = 7: 4

Skirts: blouses = 7: 2

Multiply the first by 7 and the second by 4.

Therefore:

Dresses: skirts = 49: 28

Skirts: blouses = 28: 8

[Skirts are represented by the same number: 28]

Therefore, when we combine the two ratios, we find that the ratios of dresses to blouses is 49:8