At their lowest level, computers cannot subtract, multiply, or divide. Neither can calculators. The world's largest and fastest supercomputer can only add—that's it. It performs the addition at the bit level. Binary arithmetic is the only means by which any electronic digital computing machine can perform arithmetic.
Suppose you want the computer to add seven 6s together. If you asked the computer (through programming) to perform the calculation
6 + 6 + 6 + 6 + 6 + 6 + 6
the computer would zing the answer, 42, back to you before you could say bit bucket. The computer has no problem performing addition. The problems arise when you request that the computer perform another type of calculation, such as this one:
42 – 6 – 6 – 6 ...