Gpt's code for finding the maximum of { p( c(a,key) ) } where p is english scoring function
 and c is xor decryption function.
 
 function singleByteXOR(hexString) {
    const hexToBytes = (hex) => new Uint8Array(hex.match(/.{1,2}/g).map(byte => parseInt(byte, 16)));

    const xorWithKey = (bytes, key) => bytes.map(byte => byte ^ key);

    const frequencyScore = (text) => {
        // A simple scoring function based on the frequency of common English letters
        const frequencies = {
            'a': 0.08167, 'b': 0.01492, 'c': 0.02782, 'd': 0.04253, 'e': 0.12702,
            'f': 0.02228, 'g': 0.02015, 'h': 0.06094, 'i': 0.06966, 'j': 0.00153,
            'k': 0.00772, 'l': 0.04025, 'm': 0.02406, 'n': 0.06749, 'o': 0.07507,
            'p': 0.01929, 'q': 0.00095, 'r': 0.05987, 's': 0.06327, 't': 0.09056,
            'u': 0.02758, 'v': 0.00978, 'w': 0.02360, 'x': 0.00150, 'y': 0.01974,
            'z': 0.00074, ' ': 0.13000  // Adding space for better scoring of English text
        };

        const score = text.toLowerCase().split('')
            .map(char => frequencies[char] || 0)
            .reduce((total, freq) => total + freq, 0);
        return score;
    };

    const ciphertext = hexToBytes(hexString);
    let bestScore = 0;
    let bestKey = 0;
    let bestPlaintext = '';

    for (let key = 0; key <= 255; key++) {
        const plaintextBytes = xorWithKey(ciphertext, key);
        const plaintext = String.fromCharCode(...plaintextBytes);
        const score = frequencyScore(plaintext);
        if (score > bestScore) {
            bestScore = score;
            bestKey = key;
            bestPlaintext = plaintext;
        }
    }

    return { key: bestKey, plaintext: bestPlaintext };
}

const hexString = '1b37373331363f78151b7f2b783431333d78397828372d363c78373e783a393b3736';
const result = singleByteXOR(hexString);
console.log('Key:', result.key);
console.log('Plaintext:', result.plaintext);


Another task is find the sum of multiples 
in a 1..n ranges which are 3 5 7.
Good spotting of progression formula.
x1+x2+xn = x(1+xn)/2;
find sums divisible by 3 5 7
substract the progressions of 15 21 35 105.

function sumDivisibleBy(n, divisor) {
    const p = Math.floor(n / divisor);
    return divisor * (p * (p + 1)) / 2;
}

function sumDivisibleByThreeFiveSeven(n) {
    return (
        sumDivisibleBy(n, 3) +
        sumDivisibleBy(n, 5) +
        sumDivisibleBy(n, 7) -
        sumDivisibleBy(n, 3 * 5) -
        sumDivisibleBy(n, 3 * 7) -
        sumDivisibleBy(n, 5 * 7) +
        sumDivisibleBy(n, 3 * 5 * 7)
    );
}

// Test the function
const n = 100;
console.log(sumDivisibleByThreeFiveSeven(n)); // Output: 1935