Question

Give a rough estimate (by rounding off to nearest hundreds) and also a closer estimate (by rounding off to nearest tens): $8,325-491$

Answer 1

Step $1$ : Rounding off to hundreds:

If the tens digits are $0,1,2,3$ or $4$ then round down to the previous hundred, and if the tens digits are $5,6,7,8$ or $9$ then round up to the next hundred.

Rounding off $8,325$ to the nearest hundreds$=8,300$

Rounding off $491$ to the nearest hundreds $=500$

On subtracting,

$8,300-500=7,800$

Step $2$ : Rounding off to tens:

If the ones digits are $0,1,2,3$ or $4$ then round down to the previous ten, and if the ones digits are $5,6,7,8$ or $9$ then round up to the next ten.

Rounding off $8,325$ to the nearest tens $=8,330$

Rounding off $491$ to the nearest tens $=490$

On subtracting,

$8,330-490=7,840$

Hence, using rounding off to nearest hundreds answer is $7,800$ and using rounding off to nearest tens answer is $7,840$.