中學數學[[聯立二元一次方程]](英文)

Note that 1.8kg = 1800g

Let there have A g grade A tea while grade B tea have B g :

A + B = 1800.........(1)

(120A + 84B) / (A + B) = 100.........(2)

sub (1) to (2): 120A + 84B = 180000

10A + 7B = 15000...(3)

(1)*10 - (3) :

10A + 10B - 10A - 7B = 18000 - 15000

3B = 3000

B = 1000

A = 1800 - 1000 = 800

So grade A tea have 800g in 1.8kg while grade B tea have 1000g .