Question

the simple interest received from investment is directly proportional to the amount of the investment. By investing $4000 in a municipal bond you obtain interest of $280 at the end of 1year. Find a mathematical model that gives the interest I at end of 1year in terms of the amount P.

Answer 1

Let x=interest,

4000:280 = P : x

converts into the equation

4000/280 = P / x
which when cross multiplied gives
4000x = 280P
divide both sides by 4000 to get:
x(interest) = (280/4000)P
=(7/100)P
or
=0.07P