The cost of "x" radios is $10x and transportaiton fee of $5. So cost equation for "x" radios is,

(b)
The selling price of one radio is $12. So for "x" radio Profit(selling price) equation is,

(c)
Determine the value of x for which profit equation is equal to cost equation.

The number of radios can never be in fraction. So individual has to buy 3 radios to have equal cost and profit,.