The easiest approach is to realise that one hour is 3 times longer than 20 minutes. The longer the time, the more they will pave.
215 of a mile, in 20 minutes, how much in 60 minutes?#
They will pave 3 times more.
215×31=615 of a mile
615=25 of a mile
You could also use the 'unitary method' where you find out how much they pave in ONE minute (divide by 20) and them multiply by 60 to find how much in one hour.
She has 10lbs of 25% syrup... so, in the 10lbs, 25% of that is syrup, the rest, namely the 75% remaining is water or other substances.
let's say she adds "x" lbs of water, to get "y" lbs for the 10% mixture.
how much is 25% of 10lbs? well, (25/100) * 10, or 2.5.
the water has no sugar syrup in it, so is just pure water, so the amoun of syrup in it is 0%, how much is 0% of "x" lbs? well, (0/100) * x, or 0.00x, which is just 0.
how much is 10% of "y" lbs? well (10/100) * y, or 0.10y.
whatever "x" and "y" are, we know that 10 + x = y.
we also know that the syrup amount in that is also 2.5 + 0.00x = 0.10y, thus