Suppose the father has X yuan and the son has Y yuan.

x+y= 100

5y-2x= 10

The above two equations are calculated as follows:

2x+2y=200

5y-2x= 10

These two equations add up.

2y+5y=200+ 10 7y=2 10

y=2 10/7=30，x=70 .

Hope to adopt.