Answer: 15 kg
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Step-by-step explanation:
Let x be the mass of the lead, and this mass is in kg.
The ratio 13:6:1 can be scaled up to 13x:6x:1x after multiplying all parts by x.
The portion in the middle (6x) represents how much zinc we have, while the last part (1x or simply x) is the amount of lead.
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We're told that we have 90 kg of zinc. Set this equal to 6x and solve for x
6x = 90
6x/6 = 90/6 ..... dividing both sides by 6
x = 15
So we have 15 kg of lead
Side note: we also have 13x = 13*15 = 195 kg of copper.