Solution

Total number of pens: 7
Cost of four pens costing a dollar each = 4 x  $1     =  $4
Cost of three pens costing 30c each =     3 x  $0.3  =  $0.9
Total cost of seven pens =                        $4 + $0.9  =  $4.9
Average Cost of each pen =                    $4.9 / 7      =  $0.7
 

Thus the average (mean) cost of each pen is 70 cents