There are 3 x 10 = <<3*10=30>>30 photos in total that can be placed in the first 10 pages.
There are 4 x 10 = <<4*10=40>>40 photos in total that can be placed in the next 10 pages.
A total of 10 + 10 = <<10+10=20>>20 pages of the album have been used.
A total of 30 + 40 = <<30+40=70>>70 photos can be placed on those 20 pages.
So there are still 100 - 70 = <<100-70=30>>30 photos left to be placed on the album.
And there are 30 - 20 = <<30-20=10>>10 pages left in the album.
Therefore, 30/10 = <<30/10=3>>3 photos can be placed on each of the pages.
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