Strontium-90 has a half-life of 28 days. Find the mass (in mg) remaining after 20 days. (Round your answer to one decimal place.) mg

check the picture below.

well, the angle at the vertex d is clearly not a right-angle, let's check for the one at the vertex f which does look like,   however if it's indeed, that means ef is perpendicular to df, in which case their slopes will be negative reciprocal of each other.

so the negative reciprocal of the slope of ef is 1, however the slope of df is not 1, is 1/2, so nope, they're not perpendicular lines.

is the anwser b. (-13)?

to be honest i don't know but i need points so