Rounding 24.24, you get 24. Rounding -27.8, you get -28. Therefore, 24-28=-4, so -4 is a good estimate.
3 1/2- (-1 1/2) = 3 1/2 + 1 1/2 = 4 + 1 = 5
-2.5 + (-0.5) = -2.5 - 0.5 = -3
-1/4 + (-4 3/4) = -1/4 - 4 3/4 = -4 - 1 = -5
