a+0.7=c
b+ 1.2=c
a+b=c
When the current two equations are added together, there are
a+b+0.7+ 1.2=2c
Because a+b = c
So a+b+0.7+1.2 = c+0.7+1.2 = c+1.9 = 2c.
So c = 1.9 yuan.
You can also think so.
Naughty bag to buy a ball is worse than 0.7 yuan, naughty bag money and smiling face money just buy a ball, smiling face 0.7 yuan.
Similarly, you buy a ball with a smile, which is 1.2 yuan, and you buy a ball with a smile, which is 1.2 yuan.
The ball is 1.2+0.7 = 1.9 yuan.