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Finding what Emma paid for her radio £78 / 1.3 = £60 (M1) (A1)

Finding what percentage of £80 that is £60 / £80 = 0.8 or 80% (M1)

Recognising the discount is 20% ie. 1-0.8=0.2 (A1)

If the ratio is 2 : 3 then in a class of 5 there would be 2 boys and 3 girls. 

So we can say that 2/5 of the class are boys and 3/5 of the class are girls. 

And we know that to calculate 3/5...

y = 2x + 5 y = 4, therefore 2x + 5 = 4 2x = 1 x = 1/2

a² + b² = c²

a : 9       b : unknown      c: 15

b² =c² - a²

so b² = 225 - 81

=144 so b=12

42 x 5 = 210.

45+29+63+42 = 179.

210 - 179 = 31

X = 31