Carl Icahn tweets, and Apple Inc.’s market value jumps by more than $8 billion in one day.

In a letter to Apple Chief Executive Tim Cook that Icahn made public through a tweet at just after 11 a.m. Eastern on Monday, the billionaire activist investor said he believes the technology giant’s stock is now worth $240, which is 84% above where the stock closed on Monday.

The stock was trading down a few pennies at around $128.74 at 11 a.m. With the stock closing Monday up 1.1% at $130.19, Apple’s AAPL, -3.17% market capitalization increased by about $8.35 billion since Icahn’s tweet, based on data provided by FactSet.

In addition, Icahn’s hedge fund Icahn Associates, which is Apple’s seventh largest shareholder with 52.8 million shares as of March 31, saw the value of its investment grow by $76.5 million in a day since Icahn’s tweet. Since March 31, Icahn has made $303.9 million on his Apple investment.

Icahn said he arrived at his $240-a-share valuation by estimating fiscal 2016 earnings per share of $12, applying a price-to-earnings multiple of 18 times, and then adding $24.44 of net cash per share.

The FactSet consensus EPS estimate for fiscal 2016 is currently $9.58 and Apple’s P/E ratio is 16.1, while the S&P 500’s P/E ratio is 18.6.

Icahn said he believes Apple is poised to enter, and dominate, the new TV market next year, and the automobile market by 2020, which have a combined addressable market of $2.2 trillion.

“Considering the massive scope of its growth opportunities and track record of dominating new categories, we actually think [a P/E ratio of] 18x will ultimately prove to be too conservative, especially since we view the market in general as having much lower growth prospects,” Icahn wrote in his letter to Cook.

While Icahn is pleased that Apple has repurchased $80 billion worth of its shares, and appreciates that the share repurchase program was just increased by $50 billion, but he said he believes the difference between the current stock price and what its value should be, and the fact that company’s “enormous” cash position continues to grow, indicate it’s time for a “much larger” buyback.

“Apple has clearly demonstrated a track record of excellence and success when entering new categories. We expect this to continue with the Apple Watch, the television, and the car, and the world will look back on today’s undervaluation as a fascinating example of market inefficiency (and likewise on our valuation at 18x earnings per share as conservative),” Icahn wrote.