For every 10 tarts Mrs Lee baked, 7 are egg tarts. She baked 50 tarts altogether. How many egg tarts did mrs lee bake?

Mrs. Lee baked 50 tarts / 10 = <<50/10=5>>5 sets of 10 tarts.

So, she baked a total of 5 sets x 7 egg tarts = <<5*7=35>>35 egg tarts. Answer: \boxed{35}.