https://mathweb.ru/allsystem.html?id=340003

Умножить FFFFFFFFFFFFFFFF(16) на 800000000000000(16) = 7FFFFFFFFFFFFFFF800000000000000 Столбиком

Введите два числа и укажите их основание системы счиления:

x																F	F	F	F	F	F	F	F	F	F	F	F	F	F	F	F
x																	8	0	0	0	0	0	0	0	0	0	0	0	0	0	0
+																0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
															0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
														0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
													0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
												0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
											0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
										0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
									0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
								0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
							0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
						0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
					0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
				0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
			0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0
	7	F	F	F	F	F	F	F	F	F	F	F	F	F	F	F	8
	7	F	F	F	F	F	F	F	F	F	F	F	F	F	F	F	8	0	0	0	0	0	0	0	0	0	0	0	0	0	0

Ответ: FFFFFFFFFFFFFFFF₁₆ * 800000000000000₁₆ = 7FFFFFFFFFFFFFFF800000000000000₁₆