Question 258861
Setup
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Let Jack's age = A
Let Jill's age = B

Equation 1: {{{A = B + 8}}}  (Jack is 8 years older than Jill)
Equation 2: {{{(A + 7) + (B + 7) = 64}}}  (To obtain their ages in seven years)

Solution
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Substitute equation 1 into equation 2
{{{(B + 8) + 7) + (B + 7) = 64}}} Combine like terms
{{{2B + 22 = 64}}} Subtract 22 from both sides
{{{2B = 42}}} Divide both sides by 2
{{{highlight(B = 21)}}}