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What is the highest power of 2 that divides 20! completely?

Answer
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Hint: We start solving the problem by recalling the definition of factorial(!) of a positive number n. Using this definition, we find the value of the given factorial 20!. We then take the product of even numbers and odd numbers separately. We then factorize the even numbers to get each power of 2. After factorizing every even number, we use the law am.an=am+n to get the total power 2 which will be the required highest power.

Complete step-by-step solution:
According to the problem, we need to find the highest power of 2 that divides 20! completely.
Let us first recall about the definition of factorial(!). We know that the factorial of a positive number n is denoted by n!, is the product of all positive numbers that is less than or equal to n.
i.e., n!=n×(n1)×(n2)×......×2×1. We use this definition to find the value of 20!.
So, we have 20!=20×19×18×17×16×15×14×13×12×11×10×9×8×7×6×5×4×3×2×1.
Let us take the even product of even numbers and product of odd numbers, as all the even numbers are divisible by 2.
20!=(20×18×16×14×12×10×8×6×4×2)×(19×17×15×13×11×9×7×5×3×1).
Let us factorize the even numbers present in the factorial.
20!=((22×5)×(2×9)×24×(2×7)×(22×3)×(2×5)×23×(2×3)×22×2)×(19×17×15×13×11×9×7×5×3×1).
Let us take the product of exponents of 2 separately.
20!=(22×2×24×2×22×2×23×2×22×2)×(5×9×7×3×5×3)×(19×17×15×13×11×9×7×5×3×1).
From law of exponents, we know that am.an=am+n.
20!=(22+1+4+1+2+1+3+1+2+1)×(5×9×7×3×5×3)×(19×17×15×13×11×9×7×5×3×1).
20!=(218)×(5×9×7×3×5×3)×(19×17×15×13×11×9×7×5×3×1).
Let us assume the product other than the exponent of 2 be ‘d’.
20!=(218)×d ---(1).
So, we have found that 218 divides 20! completely from equation (1), which makes 18 is the highest power of 2 that divides 20! completely.
The highest power of 2 that divides 20! completely is 18.

Note: We can also solve this by applying step function for the division of 20 and every number which is a power of 2. The process of solving can be seen as detailed as follows:
Highest power of 2 that divides 20! = [202]+[2022]+[2023]+[2024]+[2025]+.......
Highest power of 2 that divides 20! = [202]+[204]+[208]+[2016]+[2032]+.......
Highest power of 2 that divides 20! = [10]+[5]+[2.5]+[1.25]+[0.625]+.......
We know that step function takes the value of the integer that is less than or equal to the number present inside the function.
Highest power of 2 that divides 20! = 10+5+2+1+0+0.......
Highest power of 2 that divides 20! = 18.
We neglected other terms as we are getting the step function as 0.
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