Question

An or contains 15% iron. How much ore will be rsquired to get 18kg of iron

Answer 1

An ore contains 15% iron, means 1 kg of ore contains 0,15 kg of iron, therefore

1 kg ~ 0,15 kg
x kg ~ 18 kg

1/x = 0,15/18
0,15x = 18
x = 18 : 0,15
x = 120 kg

It needs 120 kg of ore to get 18 kg of iron

Answer 2

Let the total weight of the ore be x kg
therefore, the current weight of iron=15x/100 kg
so,
when the weight of iron is 15x/100 kg, the weight of ore is x kg.
or, when the weight of iron is 1 kg, the weight of ore is x*100/15x kg = 20/3 kg
or, when the weight of iron is 18 kg, the weight of ore is 20/3*18kg = 120 kg.

Therefore, it needs 120 kg of ore to get 18 kg of iron.