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What is 20 multiplied by 20?
A. 200
B. 400
C. 440
D. 420
Correct Answer Is Option = 400
Notes / Explanation
When the number twenty is multiplied by twenty, the result is four hundred, and this calculation, which is an example of a number multiplied by itself, provides a good opportunity to reflect on multiplication and on the idea of a square number. When a number is multiplied by itself, the result is called the square of that number, so that four hundred is the square of twenty, and multiplying twenty by twenty is the same as squaring twenty. Such calculations appear frequently in mathematics, for example in problems involving areas, since the area of a square shape is found by multiplying the length of its side by itself, so that a square with sides of twenty units has an area of four hundred square units. This particular calculation is quite easy to work out, because it involves round numbers. One simple way to work it out is to note that twenty is two tens, so that twenty multiplied by twenty is the same as two multiplied by two, which is four, followed by the appropriate number of zeros from the tens, giving four hundred. Another way to see it is to note that twenty multiplied by twenty can be worked out by first multiplying twenty by two to get forty, and then multiplying by ten to get four hundred, or by multiplying two by twenty to get forty and then by ten. Multiplying by numbers that are multiples of ten is made easier by the fact that one can multiply the simple numbers and then account for the zeros. A good command of multiplication, including the multiplication of round numbers and the squaring of numbers, is very useful, both for solving problems in mathematics and for dealing with many practical situations. Knowing the squares of the round numbers, such as ten, twenty, thirty and so on, is particularly useful, since these appear often and are easy to remember. Questions testing the ability to carry out such calculations are common in examinations. For PPSC and other competitive examinations, the key points to remember are that twenty multiplied by twenty equals four hundred, that a number multiplied by itself gives its square, so that four hundred is the square of twenty, that this calculation is made easy by the fact that twenty is a round number, being two tens, and that knowing the squares of the round numbers is useful because they appear often and are easy to remember.
