ex_6.js

function doubleAfter2Seconds(x) {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve(x*2);
        }, 2000);
    });
}

async function addPromise(x){
    let a, b, c;
    a = doubleAfter2Seconds(10);
    b = doubleAfter2Seconds(20);
    c = doubleAfter2Seconds(30);

    let values = await Promise.all([a, b, c]);
    return values.reduce((a, b) => (a + b), x);
}

addPromise(10).then(console.log)