And explain how to solve these problems plz 10 points

it is given that y varies directly as x. if y = 5 when x = 4 determine the constant of variation.
a.-4/5\
b.4
c.5
b.5/4

it is given that y varies directly as x. if y = 10 when x = 3 determine the constant of variation.
a.-3/10
b.10
c.3
d.10/3

which of these is true?

a.780,500 is less than 880,500.
b.780,500 is greater than 880,500.
c.880,500 is greater than 950,100.
d.880,500 is less than 780,500