That's not certain and there is a difference of opinion in legal circles. The Twelfth Amendment says "The person having the greatest number of votes for President, shall be the President, if such number be a majority of the whole number of Electors appointed; and if no person have such majority, then from the persons having the highest numbers not exceeding three on the list of those voted for as President, the House of Representatives shall choose immediately, by ballot, the President." As we all know there's 538 electors but if Congress throws out the 20 Pennsylvania votes is the majority number needed still 270 or does it drop to 260?
The argument for 270 would be that the Pennsylvania electors had been appointed and Congress had decided not to count them so the number of Electors appointed remains at 538.
The argument for 260 would be that Congress had determined the PA Electors were not validly appointed otherwise there would have been no reason to reject the votes and so the number appointed became 518.
Surely precedent can help us? Sadly not. The last time a state's electors were thrown out was 1872. At the time there was 366 electors but Arkansas and Louisiana's 14 electors were thrown out for allegations of electoral fraud. The Journal of the House of Representatives says that the majority needed was 177 but the Journal of the Senate says the majority needed was 184. As Grant received 286 it didn't matter. After the mess of 1876, the Electoral Count Act of 1877 should have sorted it out but Congress decided to punt the issue away. It is entirely possible in a close election that the House could determine a candidate reached the majority and the Senate decide that they didn't have that majority.