To make this a bit easier we can convert the fractions to decimals, then add them up to find out how much flour they originally had. Rashan bought 3 7/10 pounds of flour, which becomes 3.7 pounds. Diego bought 2 3/4 pounds of flour, which becomes 2.75 pounds of flour. In total they both had 3.7 + 2.75 = 6.45 pounds of flour.
Now they used 4 3/5 pounds to bake some bread, which becomes 4.6 pounds. To find out how much flour they have left, we subtract the 4.6 from 6.45, which equals 1.85 pounds of flour left.