Investor owns 20 shares, each paying $61; so total dividends received is
20 x 61 = 1,220.
Next figure out how many total dollars Investor has in the stock... it's an amount such that the $1,220 represents a 6.1% return. Using the standard 'return on investment' equation...
... you just need to solve for that denominator.
Now that you know the total $s invested in the stock, you can deduct from that total the first three $ amounts invested to get the fourth investment cost. Wrap it up by dividing that by the five shares acquired to get the per-share amount.