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In IEEE $754$ single floating point format, how many numbers can be represented in the interval [10, 16)?

A. $2^{21}$

B. $3 * 2^{21}$

C. $5 * 2^{21}$

D. $2^{22}$

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10 is written as

0 10000010 01<21 0's>

These rightmost 21 0's are actually redundant, and we can set them to anything.

16 is written as

0 10000011 <23 0's>

If we keep the exponent to 3 (to make add the manipulatable bits shift to one end)

0 10000010 1<22 0's>

These rightmost 22 0's are manipulatable.

So, $2^{21}+2^{22}=2^{21}(1+2)=3*2^{21}$

In the IEEE 754 notation , the number is represented in 32 bits . The first bit is for the sign of the number , next 23 bits is normalise mantissa and next 8 bits is biased exponent.

$10$ is represented as = $1.01*2^{3} = 1.01*2^{130-127}$ , which in 32 bit format would be represented as ,

$001(19 \ 0's)10000010$.

And 16 is represented as $1.0*2^4 = 1.0*2^{131-127}=0(23 \ 0's)10000011$.

Thus we need to find the number of possible patterns of $0's$ and $1's$ in the following format.

Keeping the exponent same , the number of patterns of mantissa of the form $01(21 \ x's)$ and $1(22 \ x's)$.Where $x's$ are the positions to fill with 0's and 1's .

Thus total number of possible patterns = $2^{21}(of \ the \ 1^{st} \ kind)+2^{22}(of \ the \ 2^{nd} \ kind)$ = $3*2^{21}(Ans)$.

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Why you are taking 1(22 x's) ? It should not be 2^23 as we have complete 23 zeros to select from?
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1.00000...000 x 2^3 is  how much?