Question 153913
{{{d}}}= Dave's age now
{{{h}}}= Harry's age now
{{{t}}}= Tom's age now
(1) {{{h = 3d - 17}}}
(2) {{{t = 2h + 12}}}
(3) {{{d = h - 7}}}
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Replace {{{d}}} in (1) with {{{d}}} in (3)
(1) {{{h = 3d - 17}}}
{{{h = 3*(h - 7) - 17}}}
{{{h = 3h - 21 - 17}}}
Subtract {{{3h}}} from both sides
{{{-2h = - 38}}}
{{{h = 19}}}
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(3) {{{d = h - 7}}}
{{{d = 19 - 7}}}
{{{d = 12}}}
There are 12 candles on Dave's cake
Note that I didn't need to know the father's age to solve this
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The 2nd one may be too tough for me