Since 1sand=3.5compost you can set up the ratio 1/3.5 or 1 sand for every 3.5 compost. If you change the number of sand to three, then you multiply the top by three since 1*3=3. To keep the mixed number equal you would have to multiply the bottom number by three as well. This means that you can multiple all the sand by 3.5 to get how much compost is needed.
