Find the ratio of a to b if a= (5c) and b= 10(2c)

Based on what i know about mean, this doesn't add up.

Find the number of positive three-digit even integers whose digits are among 9, 8, 7,5, 3, and 1.

Find the volume of the described solid. the solid lies between planes perpendicular to the x-axis at x = 0 and x=7. the cross sectionsperpendicular to the x-axis between these planes are squares whose bases run from the parabolay=-31x to the parabola y = 30/x. a) 441b) 147c) 864d) 882