Question

Please solve these coin problems:

Answer 1

1)
Solving from system of equations:
x- amount of 1 cent stamps
y- amount of 33 cent stamps
$\left \{ {{1*x+33*y=18,40} \atop {x+y=80}} \right.\\x=80-y\\0,01*(80-y)+0,33y=18,40\\0,8-0,01y+0,33y=18,40\\0,32y=18,40-0,8\\0,32y=17,60 /:0,32\\y=55\\x=80-55=25$
2)
x- amount of 20 cent stamops
y-amount of 34 cent stamps
From system of equations:
$\left \{ {{0,2x+0,34y=19,90} \atop {x+y=75}} \right.\\x=75-y\\0,2(75-y)+0,34y=19,90\\ 15-0,2y+0 ,34y=19,90\\-0,2y+0,34y=19,90-15\\0,14y=4,90/:0,14\\ y=35\\x=75-35=40$