Soroban abacus simulator

An interactive Japanese soroban you can actually use. Click the beads across thirteen columns — one heaven bead worth five and four earth beads worth one each — and the decimal value updates live beneath. Clear it, roll a random number, type a value to set the beads, or solve a quick addition, subtraction, multiplication or division and show the answer on the frame.

Reading a number written another way: a Roman numeral on a monument, a hexadecimal colour, a binary byte, or a fraction that needs reducing.

Assuming a decimal fraction survives a change of base. A tenth is exact in decimal and infinitely repeating in binary, which is why 0.1 + 0.2 is not 0.3 in most programming languages.

They sit next to each other but answer different questions: RSA encryption simulator is the one to open when you need it to walk through the RSA public-key cryptosystem end to end with your own primes. Enter two primes p and q and a numeric message m: the simulator computes the modulus n = pq and the totient φ = (p−1)(q−1), picks or validates a public exponent e coprime to φ, derives the private exponent d = e⁻¹ mod φ, then encrypts c = mᵉ mod n and decrypts back mᵈ… = m — every step shown with big-integer arithmetic. The definitive way to see why RSA actually works. Pick whichever matches what you're starting from — both are free.

Logic Gate Simulator is the closest one after this: Draws the gate diagram for a boolean expression and lets you flip each input to watch the signal travel through. Half-adder, multiplexer, parity and majority circuits are one click away.

Base conversion and number-theory results are exact. Roman numerals follow the standard subtractive form used since the Middle Ages, which is not the only one the Romans themselves used.