You have a fixed length of wire and need to design a generator that will produce the greatest peak emf for a given frequency and magnetic field strength. You should use (a) a one-turn square coil, (b) a two-turn square coil, (c) either a one- or a two-turn square coil because both give the same peak emf for a given frequency and magnetic field strength.

Explanation:

flux ∝ n of turns x area

in case of one turn :

area =( L/4 )²

= L² / 16

flux ∝ 1 x  L² / 16

∝   L² / 16

In case of two turns

area =( L/8)²

= L² / 64

Flux  ∝ 2 x L² / 64

∝ L² / 32 .

Since in 2 turn case flux is less , for maximum peak emf we need one turn square coil.

the old price per ounce is c/4.

the new price per ounce is c/3.6.

the ratio of the old price to the new one is

(c/4)/(c/3.6) = (c/c)×(3.6/4) = 0.9 . . = 9/10 . . "nine tenths" in either form