The relatively dark blotches on the surface of the sun are called

The relatively dark blotches on the surface of the sun are called sunspots.

Sunspots are temporary phenomena on the photosphere of the Sun that appear as dark spots compared with surrounding regions. They are areas of reduced surface temperature caused by concentrations of magnetic field flux that inhibit convection.

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