You and your friend's lunch cost $19. your lunch cost $3 more than your friend's. write an equation.

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

Peter measures the angles in a triangle. he finds that the angles are 95, 10 and 75. is he correct? explain your answer

The volume of a spherical hot air balloon v(r) = 4 3 ? r3 changes as its radius changes. the radius is a function of time given by r(t) = 3t. find the instantaneous rate of change of the volume with respect to t at t = 2.