The equation is simple. Suppose the original tea has x grams, then the milk has 4X grams. The equation is 4X+ 100+X+ 100=700.

The milk with X= 100 is 4× 100 = 400g.

There are 100+ 100 = 200g tea, and 400+ 100 = 500g milk.

Now the ratio of milk to tea is 500:: 200 = 5: 2, so the taste of tea is very heavy now.

Can you understand/?