A shallow hole is dug and lined with concrete. A number of large black plastic bags, each filled with several centimeters of water, are placed in the hole and then covered with fiberglass insulation panels. The panels let sunlight in but keep most of the heat stored in the water during the daytime from being lost to the atmosphere. This heat can then be used to heat water and space. It cannot be used to generate electricity. This technology is called:

Freshwater solar ponds or freshwater floating collector type solar pond.

Explanation:

A shallow hole is dug and lined with concrete. A number of large black plastic bags, each filled with several centimeters of water, are placed in the hole and then covered with fiberglass insulation panels. The panels let sunlight in but keep most of the heat stored in the water during the daytime from being lost to the atmosphere. This heat can then be used to heat water and space. It cannot be used to generate electricity. This technology is called freshwater solar ponds or freshwater floating collector type solar pond.

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

+15-35=-20

going in opposite directions