Estimate (845.7456)x(234.789)x(89.5) by front end rounding

845.7456 is about in the middle between 800 and 900 

234.789 is about in the middle between 200 and 300

So their product is somewhere between 800x300 (or 240000)
and 900x200 (or 180000),

So we'll say the product of the first two numbers is 
about 200000.

Then the third number 89.5 is about 90.

So the answer is about 200000 times 90 or 18000000

[The actual answer using a calculator is 17772172.85,
so our ballpark answer is correct to two figures.]

Edwin