John Merline writes for Investor’s Business Daily that billionaire Donald Trump seems to have some problems with simple computation skills.

For a person who claims to be an incredible businessman, Donald Trump sometimes seems to have difficulty handling simple addition and subtraction.

During the Republican debate in Detroit on Thursday, Chris Wallace asked Trump about his tax plan, saying that an independent analysis found it would add $10 trillion to the nation’s debt in 10 years “even if the economy grows the way you say it will.”

Trump argued that he can cover that mainly by cutting waste, fraud and abuse — which is an impossibility. Even the toughest budget hawks don’t claim there’s that much fat.

Trump also argued that the government can save “hundreds of billions of dollars in waste” because “the pharmaceutical companies are not mandated to bid properly.”

By this, Trump means that the government doesn’t directly negotiate with drug companies for Medicare’s drug benefit.

But as Wallace pointed out, “Medicare only spends $78 billion a year on drugs.”

To which Trump responded: “I’m saying saving through negotiation throughout the economy, you will save $300 billion a year.”

Even if that is what he meant, the entire country spends less than $300 billion a year on prescription drugs. In 2014, the last year for which official data are available, retail drug costs added up to $297.7 billion, according to the Centers for Medicare and Medicaid Services.

So either Trump is saying that he can negotiate the price of drugs down to less than zero, or he doesn’t know what he’s talking about, or he doesn’t care.