What explosion in the upper solar atmosphere releases about as much energy as millions of 100-megaton hydrogen bombs exploding simultaneously?

Solar flare

Explanation:

In the Sun when there is an sudden increase in brightness which is generally accompanied by a coronal mass ejection it is known as a solar flare. Solar flares occur close to Sun spot.

Coronal mass ejection is the release of matter (plasma) with a strong magnetic field from the corona of the Sun.

The energy they release ranges between which is comparable to 100-megaton hydrogen bombs exploding simultaneously

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