# Maths - averages, percentages & product of primes

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• Averages, Percentages, Product of primes.
• Averages
• Add up all of the numbers, then divide by how many there are.
• E.g: 6,7,11     Add the numbers: 6+11+7 = 24   Divide by how many number there are:            24 / 3 = 8       The mean is 8.
• E.g: 42,106,89+1    Add the numbers:     42+106+89+1  = 238           Divide by how many numbers there are:                234 / 4 = 59.5
• Percentages
• Percent means 'out of 100'.                E.g: 1/100
• E.g:    Calculate 25% of 80. 25% = 25/100.         And 25/100 x 100 = 20.           So 25% of 80 is 20.
• Reverse percentages
• E.g: In a sale, everything has been reduced by 20%. Kate buys a top for £64. How much was it before it was reduced?
• Workings out:  100-20 = 80.   64 / 0.8 = £80
• E.g: 'l' is 30% of 'k'.           'k' is 90% of 'a'.          What percentage of 'a' is 'l'?
• Workings out: 'l' = 30/100 x 'k'.               'k' = 90/100 x 'a'.
• Product of primes
• E.g: Express 72 as a product of primes.
• 72
• 36
• 6         6
• 3      2
• 3       2
• 2
• 3x3x2x2x2
• 2 (little 3) x 3 (little 2)