Question 181596
{{{a}}}= number of .25 stamps
{{{b}}} = number of .40 stamps
given:
(1) {{{a + b = 15}}}
(2) {{{.25a + .4b = 4.95}}}
Multiply both sides of (1) by {{{.4}}}
and subtract (2) from (1)
(1) {{{.4a .4b = 6}}}
(2) {{{-.25a - .4b = -4.95}}}
{{{.15a = 1.05}}}
{{{a = 7}}}
and, since
(1) {{{a + b = 15}}}
{{{7 + b = 15}}}
{{{b = 8}}}
He bought {{{7}}} .25 stamps and {{{8}}} .40 stamps
check:
(2) {{{.25a + .4b = 4.95}}}
(2) {{{.25*7 + .4*8 = 4.95}}}
{{{1.75 + 3.2 = 4.95}}}
{{{4.95 = 4.95}}}
OK