Problem
Together the faucets give the sum of their rates each minute. Change 1 3/4 to a fraction (7/4 = 14/8) so the denominators match, then add.
Two flows into one bucket simply add; matching denominators is the grade-5 way to add eighths and quarters.
5.NF.A.1Identify SubproblemsRunning together, the two faucets pour 21/8 L into the container each minute.
Both streams run at once into the same container, so what the container gains in a minute is the two amounts put together.
The two rates can only be added once they are counted in the same size of share, which is why the mixed number is rewritten in eighths.
Quarters and eighths measure the same litre, and one quarter is two eighths, so both rates can be told in one size of piece.
60 seconds make a minute, so 20 seconds is one third and the time is 16/3 minutes.
Seconds are a smaller unit than minutes; a fraction lets us name the leftover 20 seconds as part of one minute so the units agree.
5.MD.A.1Analyze The UnitsLitres-per-minute times minutes leaves litres: 21/8 x 16/3 = 14 L.
Rate times time is total amount; the units guide the setup so the answer comes out in liters.
5.NF.B.6Analyze The UnitsWhen you multiply a per-minute rate by time, write everything in the same unit first — then the units tell you the answer is liters.