At the Sphinx Elementary School, an accountant from the Central Bank was manning the polling place and handing out ballots to registered voters.

"All the Egyptian people are intelligent," he told visitors, "so of course they want peace. That means they will all vote 'yes.' And the Egyptian people all love President Carter too-put that in your report."

As Egyptians voted today in a referendum on the peace treaty with Israel and on consitutionl amendments proposed by President Anwar Sadat, there was no doubt that the accountant's prediction about the outcome would prove correct. More than 90 percent, perhaps 98 percent, of those who marked ballots were considered certain to endorse the treaty and amendments, validating the banner headline in line this morning's edition of the newspaper Al Ahram: "Millions og voters say yes today."

It was less certain whether enough of Egypt's 10.3 million eligible voters would go to the polls to enable Sadat to claim the overwheming mandate for peace and constitutional change that he is seeking.

Males theoretically are obliged to vote. But the penalty for staying away is a token fine that is seldom actually imposed, and through much of the day the turnout was slim at polling places around the city.At a polling place for women, who are not required to vote there no voters at all when reporters dropped in to check the turnout.

Voting generally was orderly and the country was mostly quiet. But the day was marred by one serious terrorist incident of the kind Egypt has feared since Sadat sidned the treaty last month. A woman customs inspector was killed and fourt other persons were wounded when a parcel she was examining at Cairo's central psot office blew up, police said.

A little-known Palestinian commando organization, the Eagles of the Palestine Revolution, claimed respondibility for planting the booby-trapped box of cigars that exploded, Reuter reported. The group issued a statement saying that all members of the unit returned safely to base following the explosion.

[The same organization claimed responsibility claimed responsibility for planting explosive charges at the Israeli diplomatic mission in Cyprus and at an office of Egypt's National Airlines in Nicosia April 5.]

Sadat, who is under attack throughout the Arab would for signing the peace treaty, called for the referendum to show his Arab critics that the people of Egypt support his policy. There is no doubt that most do, but it is not clear that the referndum will provide sufficient proof to comvince the skeptics.

Voters interviewed at random today all said that of course they had voted yes. But traditionally those who oppose Sadat have refrained from voting in his referedums. In addition, the mechancis of balloting strongly favor the government position.

Ballots are marked in secret and dropped into closed boxes. But in the poorer districts of the cities and in the countryside, the high percentage of illiteracy and unfamiliarity with elections require voters to turn to election officails for help in marking their ballots. In some villages, the inhabitants simply turn over their identity cards to a local elder who goes to the polls and votes all their proxies.

Each voter today was asked to answer tow questions:

Do you approve the peace treaty and its annexes between the Arab Republic of Egypt and the state of Israel, and the special agreement for setting up full autonomous rule in the West Bank and Gaza Strip?

Do you approve dissolution of the present People's Assembly [parliament] and the principles for re-organizing anthe state mentioned in Presidential Decree 157 for 1979?

For each question, the ballot carried a red circle to be marked for yes, a black for no.

"Of course I voted yes on both parts," said a government clerk after folding his ballot and dropping it into the box. "So is everybody."

Voters unfamiliar with Presidential Decree 157 could find it posted on walls outside polling places. The package of organizational changes and constitutional amendments was proposed by Sadat as part of the country's transition to peace. But its real significance is unclear, even to experienced political observers and sophisticated Egyptians.

It calls for dissoulution of the present parliament, new elections, freedom to establish new political parties, creation of a national consultative council of dignitaries and establishment of the government. CAPTION: Picture, President Sadat of Egypt votes in peace referendum in his native village. UPI