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Mathematical problem ball
Naughty has A yuan, smiling has B yuan, 1 ball needs C yuan.

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.