Platinum Equity increased its offer for acquiring testing and computer equipment supplier Electro Rent Corp. by 18 percent, valuing the deal at about $382 million. The Beverly Hills private investment firm took the step after an unidentified third party made an unsolicited bid to buy the Van Nuys company. Platinum will now pay $15.50 per share as of June 24 instead of the $13.12 originally offered when the transaction was announced a month ago. That represents a 47 percent premium over the closing stock price on May 27, the last day of trading prior to the original announcement of the Platinum Equity offer. News of the acquisition came less than three weeks after Daniel Greenberg announced he was retiring as Electro Rent’s chief executive in early July after 35 years. Greenberg and a member of his family own 29 percent of the outstanding shares and support the sale. Platinum Equity has more than $6 billion in assets and has a portfolio of companies in the hospitality, manufacturing, entertainment, information technology and telecom industries. Shares of Electro Rent closed down 3 cents or a fraction of a percent to $15.40 on the Nasdaq.