Proportion scale exercise answers:

1. Original photo size: 8 x 10, vertical

Reproduction width needed:

21 picas =3 1/2 inches (6 picas to an inch)

So:
8 x 10=
3 1/2 x 4 3/8

Reproduction percentage: 44 %.

2. Original photo size: 4 x 6, horizontal

Reproduction width needed: 3 3/4 inches

So:
4 x 6=
2 1/2 x 3 3/4

Reproduction percentage: 62 %.