Answer :
hello
let's allow x represent the number of 39 cent stamp
let's allow y represent the numbers of 24 cent stamp
40 = x + y .... equation 1
this equation represents the total number of stamos
14.40 = 0.39x + 0.24y ....equation 2
this equation represents the summation cost of each type of stamp
we can proceed to solve these set of equations
from equation 1
40 = x + y
let's make x the subject of formula
[tex]\begin{gathered} 40=x+y \\ x=40-y \end{gathered}[/tex]x = 40 - y .....equation 3
now we csn substitute equation 3 into equation 2 and solve
[tex]\begin{gathered} 14.40=0.39x+0.24y \\ x=40-y \\ 14.40=0.39(40-y)+0.24y \\ 14.40=\mleft\lbrace(0.39\times40)-(0.39\times y)\mright\rbrace+0.24y \\ 14.40=15.6-0.39y+0.24y \\ 14.40=15.6-0.15y \\ 0.15y=15.6-14.40 \\ 0.15y=1.2 \\ y=\frac{1.2}{0.15} \\ y=8 \end{gathered}[/tex]y = 8
put y = 8 in either equation 1 or 2
from equation 1
[tex]\begin{gathered} 40=x+y \\ y=8 \\ 40=x+8 \\ x=40-8 \\ x=32 \end{gathered}[/tex]