Find the lower bound estimate for 63.75 + 3.6

I'd be more comfortable finding the upper bound estimate, which would be

64 + 4 = 68.  63.75 would normally be rounded off to 64 and 3.6 to 4.

If you want the lower bound estimate, try 63 + 3, or 66.  Although not all that far from the actual value of   63.75 + 3.6, which is 67.35, I'm queazy about rounding 63.75 down to 63 when the normal rule would be to round it up.

Results:  lower estimate:  66
Actual sum:                       67.35
Upper estimate:                68

Subtracting is the opposite of adding