Number Sense Worksheet With Answer Key - 2008/2009 printable pdf download

Number Sense Worksheet With Answer Key - 2008/2009

Number Sense

2008/2009 Circle 2 Problem 5

Problem

On the hundred chart below:

• Cross oﬀ the number 1.

• Leaving the number 2, cross oﬀ all multiples of 2.

• Leaving the number 3, cross oﬀ all multiples of 3.

• Leaving the number 5, cross oﬀ all multiples of 5.

• Leaving the number 7, cross oﬀ all multiples of 7.

To the right of the chart, make a list of all the numbers that are NOT crossed out.

HINT: There should be 24 numbers in your list.

Hundred Chart

List of Remaining Numbers

a) Find the factors of each number in your ’List of Remaining Numbers’. What do they have in

common? Numbers (other than 1) which have this property are called prime numbers.

b) The digit sum of a number is obtained by repeatedly adding digits until a single digit remains.

For example, the digit sum of 37 is obtained as follows: 37

3 + 7 = 10

1 + 0 = 1 , giving

a digit sum of 1.

(i) How many of the numbers in your list above have a digit sum of 2?

(ii) How many of these number have a digit sum of 6?

c) Consider the numbers from 1 to 99.

(i) Explain why, if one of those numbers has a digit sum of 6, then the sum of its digits must

equal to 6 or to 15.

(ii) Write down all numbers from 1 to 99 that have a digit sum of 6.

Number Sense Worksheet With Answer Key - 2008/2009