Which of the following is the best estimate of 5 and three-eighths minus 1 and nine-tenths?

Question

anonymous · Answer

This might depend on the required level of accuracy.  

Start with a sort of gross estimate.  5 and a half minus two is about 3 and a half. 

 We can refine that a little by saying our starting number was an eighth less than our rounded starting number, but we only took away a tenth less than the actual value.  The difference between an eighth and a tenth is pretty small (about a fortieth) so we might be happy with our original estimate. 

 If we want to really refine it, we will find the exact answer to be a fortieth less than three and a half, or three and nineteen fortieths.