UPSC CDS Math Linear Equations in One and Two Variables Questions

Let the number of sweets costing ₹7 and ₹10 be x and y, respectively.
7x+10y=200
⇒y=

200−7x

10

⇒y=20−

7x

10

Here, x and y are positive integers.
For y to be a positive integer, x=10 or 20
When x=10 or 20.
When x=10,y=20−7=13
When x=20,y=20−14=6
Therefore, the maximum number of sweets he can get =20+6=26