Question

The weight of Jane and Jessica was in the ratio of 8 : 9. Jane gained 2 kg while Jessica lost 4 kg. They then had the same weight. What was Jane's weight at first?

Answer 1

48 kg

Explanation:

The given relations can be used to write a system of equations for the two weights. Those can be solved to find Jane's weight.

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setup

Let x and y represent Jane's and Jessica's original weight, respectively. The ratio of weights was ...

x/y = 8/9

After the changes in weight, they were equal:

x+2 = y-4

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solution

Adding 4 to the second equation, we have an expression for y that can be substituted into the first equation.

y = x +6 . . . . . . . . . . solve the second equation for y

x/(x+6) = 8/9 . . . . . . substitute for y in the first equation

9x = 8(x +6) . . . . . . . multiply by 9(x+6)

x = 48 . . . . . . . . . . . simplify and subtract 8x

Jane weighed 48 kg at first.

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Alternate solution

The original difference in "ratio units" was 9-8 = 1 ratio unit. We find that this corresponds to 6 kg after the weight changes make the weights equal. Then 8 ratio units will be 8(6 kg) = 48 kg—Jane's original weight.

(This mental solution is virtually the same as the solution using equations shown above.)