Question 180802
Let the number of $.34 stamps = {{{a}}}
Let the number of $.23 stamps = {{{b}}}
given:
(1) {{{a + b = 15}}}
(2) {{{.34a + .23b = 4.44}}}
Multiply both sides of (1) by {{{.23}}} and 
subtract from (2)
(2) {{{.34a + .23b = 4.44}}}
(1) {{{-.23a - .23b = -.23*15}}}
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(2) {{{.34a + .23b = 4.44}}}
(1) {{{-.23a - .23b = -3.45}}}
{{{.11a = .99}}}
{{{a = 9}}}
And, since
(1) {{{a + b = 15}}}
{{{b = 15 - a}}}
{{{b = 6}}}
9 of the $.34 stamps were sold and
6 of the $.23 stamps were sold
check:
(2) {{{.34a + .23b = 4.44}}}
{{{.34*9 + .23*6 = 4.44}}}
{{{3.06 + 1.38 = 4.44}}}
{{{4.44 = 4.44}}}
OK