Whoa ! Before we talk about 'how', we need to agree on 'what'.
Are you saying that you have the problem ' 6 x 3/2 ', and you know that the answer is ' 21 ' and you want to know how the answer was worked out ?
You can stop right there.
To multiply a fraction by a whole number, just multiply the numerator of the fraction by the whole number. Then simplify the fraction if it needs simplifying.
6 x 3/2 = (6 x 3) / 2 = 18/2 .
To simplify 18/2 , divide top and bottom by 2. Then you have ' 9 ' , and that's the answer.
You dont even have to do the math, but each second is a 32 distance increase. Just count in your head to 6.
And notice how the third one is 80 so double that to 6 is 160, but plus the first second (16) since we are not starting from 0. which ends up to be 176