Question 352560
Let {{{p}}} = Peter's age
Let {{{s}}} = Sharon's age
Let {{{j}}} = John's age
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given:
(1) {{{p = s + 7}}}
(2) {{{j = 2p}}}
(3) {{{(p + s + j)/3 = 19}}}
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From (1):
{{{p = s + 7}}}
{{{s = p - 7}}}
Substitute (1) and (2) in (3)
(3) {{{(p + s + j)/3 = 19}}}
{{{(p + p - 7 + 2p)/3 = 19}}}
{{{(4p - 7)/3 = 19}}}
{{{4p - 7 = 57}}}
{{{4p = 64}}}
{{{p = 16}}}
Peter is 16
check answer:
{{{s = p - 7}}}
{{{s = 9}}}
and
{{{j = 2p}}}
{{{j = 32}}}
and
(3) {{{(p + s + j)/3 = 19}}}
(3) {{{(16 + 9 + 32)/3 = 19}}} 
{{{57/3 = 19}}}
{{{57 = 57}}}
OK