Posted by B.B. on .
Here is the question: Give an estimate for the following expression. Then state whether you believe that the exact answer is greater than or less than the estimate. Explain how you decided, The original question is this 54*23. Here is what I've answered: 50*20. I rounded down. 54 went to 50 and 23 to 20. 54*23=1242 50*20=1000. The exact answer is greater than the estimate. I rounded 12 to 10 and 26 to 30.
Yes, your method satisfies what the teacher needs, namely give an estimate of the value, and be able to state whether the exact answer is higher or lower than your estimate.
If you want to have a closer estimate (or even the exact answer), here's what I would do, knowing that you already know how to multiply a number by 11 mentally.
=55*23 - 1*23
=11*(23*5) - 23
=11*115 - 23
1265 is the estimate, it is an over estimate.
The exact answer can also be calculated mentally as 1265-23=1242
Note: To multiply a number by 5, add a zero and divide by 2.
23*5 = 230/2 =115