Question 957149
{{{X}}} - number of P10 coins
{{{Y}}} - number of P5 coins
.
.
.
{{{X+Y=75}}}
{{{10X+5Y=450}}}
Multiply first equation by 10 and subtract from second equation.
{{{10X+5Y-10X-10Y=450-750}}}
{{{-5Y=-300}}}
{{{Y=60}}}
Then,
{{{X+60=75}}}
{{{X=15}}}