Question

10 glasses cost 40 dollars. How much do 20 glasses cost?

Answer 1

10 glass=40$
1 glass = 40 /10
20 glasses = 40/10 x20
                  = 4x20
                  = 80 $

Answer 2

given 10 glasses cost 40 dollars
therefore we can write
10----40. 1 equation
and for 20 glasses......
20------? 2 equation
from 1 & 2
10----40
20----? (cross multiplication)
? equals 40#20 /10
80
80 dollers will be for 20 glasses