Step-by-step explanation:
We can write a system of linear equations with this information.
• x: number of 25 cent stamps
,
• y: number of 20 cent stamps
The system is:

Solving using the substitution method, we first clear y from the first equation:

Replace into the second equation:

And solve for x:

Finally, replace x = 32 into the equation for y we cleared before to find the value of y:

Answer:
He bought:
• 32, ,25-cent stamps
,
• 8 ,20-cent stamps