UPSC CDS 2 2021 Math Paper

Let the cost of one pencil, one notebook and one eraser be x,y and z respectively.
Then, according to the question,
‌5x+6y+7z=250 . . . (i)
‌ and ‌6x+4y+2z=180 . . . (ii)
‌6×‌ Eq. (i) ‌−5×‌ Eq. (ii) ‌
‌30x+36y+42z=1500
‌30x+20y+10z=900
dividing the equation by 8 , we get
2y+4z=75
Hence, the cost of 2 notebooks and 4 erasers is ₹75.