Answer:
x=9, y=10. (9, 10). The two numbers are 9 and 10.
Step-by-step explanation:
3x+5y=77
x+y=19
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3x+5y=77
-3(x+y)=-3(19)
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3x+5y=77
-3x-3y=-57
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2y=20
y=20/2
y=10
x+10=19
x=19-10
x=9
This is a units conversion problem: you have time and you want to convert that to money. Any units conversion problem involves multiplication by 1 in various forms. When you are given 7.5 min = 1 bottle = 500g, you know that (500 g)/(7.5 min) = 1 because the numerator and denominator are equal. (Of course, you are aware that 500 g = 0.500 kg.) The idea is to choose the forms of 1 that cause units you don't want to cancel, leaving only the units you do want.

The glass for recycling that would save enough energy to power an oven for 6 hours costs 120p.