Sometimes
in order to solve problems with large numbers, it is necessary to use a two digit
divisor.
In order to
divide by two digits, let's first review rounding. It is
necessary to have that starting point when dividing large numbers.
Quick review of
terms:
"Where to
start?" Look at the original problem. Round the
divisor to the nearest tens' place to
get a place to start. (If you need to review rounding rules,
click here.)
Since 43 rounds
to 40, think "How many 40s go into 88?"
Since that
answer is "2", divide 88 by 43 and put the product of 2 and 43 under
the dividend.
Then subtract.
The answer is 2 R2.
86 2
Check your
answer by multiplying, and adding the remainder. Your answer
should be the original dividend.
43 x 2
86 + 2
88
Review:
To get started,
round the divisor to the nearest tens' place.
Ask yourself the
question, "How many times does the rounded number divide into
the dividend?" That is the quotient.
Then using the
original divisor (not the rounded one), multiply that divisor by the
quotient. Put that product under the dividend.
Subtract that product from the
dividend. That difference is the remainder.
Check your answer by multiplying
the quotient by the divisor. If there is a remainder, add that
to the product. The final answer of your check should be the
same as the dividend.
Try some!
Round to the nearest ten.
88
93
41
37
8
45
11
29
Now, use those rounded numbers to
answer these.
In the problem
78 ÷ 11, the best estimate is ?
In the problem
61 ÷ 29, the best estimate is ?
In the problem
89 ÷ 37 the best estimate is ?
Remember, the rounding of the divisor
is only an estimate of where to begin. It doesn't always work out!
Look at this problem:
In this
problem, round the divisor.
The divisor 8
rounds to 10 and goes into the dividend 2 times.
But when you go
back to the original divisor, 8, and multiply it by the 2(estimate),
the product is too small because the
remainder is too large, (larger than the divisor and that cannot
be!). So you need to use one larger than 2, which is 3.
16 11
So remember the
rounding is just a beginning, you must "adjust" the answer after
that to make it correct.
