Question 705286
Let B = Billy's Age
Let D = Dave's Age
Let J = Jon's Age
Equation 1: {{{D = B + 4}}}
Equation 2: {{{B = 3J}}}
Equation 3: {{{B + D + J = 39}}}
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Rewrite equation 2 so that it is solved for J
{{{B/3 = J}}}
Now both equation 1 and equation 2 are in terms of B
Substitue those values into equation 3 for the same letter.
Equation 3: {{{B + D + J = 39}}}
{{{B + (B + 4) + (B/3) = 39}}}
Multiply everything by 3 to remove the denominator
{{{3B + 3B + 12 + B = 117}}}
Combine like terms
{{{7B + 12 = 117}}}
Subtract 12 from both sides
{{{7B = 105}}}
Divide both sides by 7
{{{highlight(B = 15)}}}
Now plug 15 into the equations 1 and 2 for B.
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Equation 1: {{{D = B + 4}}}
{{{D = (15) + 4}}}
{{{highlight_green (D = 19)}}}
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{{{B/3 = J}}}
{{{(15)/3 = J}}}
{{{highlight(5 = J)}}}