Help please math people

The? half-life of a radioactive substance is 20 years. if you start with some amount of this? substance, what fraction will remain in 120 ? years

X^2/100+y^2/25=1 the length of the major axis is: a)5 b)10 c)20

Indicate the equation of the given line in standard form. the line containing the longer diagonal of a quadrilateral whose vertices are a (2, 2), b(-2, -2), c(1, -1), and d(6, 4).