A reaction is half complete after 20 minutes. After 40 minutes the reaction is two-thirds complete. When will the reaction be 90% complete?

because u need two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei to form isotopes.

so since they all have the same shape? i mean maybe because m& m and reeses are both chocolates

it's c (engineer's do their work with mechanical stuff not on any living being)