How many kgs of rice of variety 1 costing Rs 42 kg should a shopkeeper mix with 25 kgs of rice of va...........

Explanation:

25% profit by selling the mixture at Rs.40/kg,
his cost per kg of the mixture = Rs.32/kg.
(x×42)+(25×24)=32(x+25)
42x+600=32x+800
10x=200
x=20kgs.
Shortcut:
125 = 40
100 = 32
42……………….24
………32…………..
8…………………….10
10 = 25
8 = 20