Math~

Mister Adam sells tapioca pearls in 4 kilo and 3 kilo containers. With much demand for the tapioca pearls, he sold 30 containers weighing 100 kilos. How many of each type did he sell?

Let f=4 kilo containers

Let t=3 kilo containers

f + t=30, and,

4f + 3t=100

Solve the following system:
{f+t = 30 |     (equation 1)
4 f+3 t = 100 |     (equation 2)
Swap equation 1 with equation 2:
{4 f+3 t = 100 |     (equation 1)
f+t = 30 |     (equation 2)
Subtract 1/4 × (equation 1) from equation 2:
{4 f+3 t = 100 |     (equation 1)
0 f+t/4 = 5 |     (equation 2)
Multiply equation 2 by 4:
{4 f+3 t = 100 |     (equation 1)
0 f+t = 20 |     (equation 2)
Subtract 3 × (equation 2) from equation 1:
{4 f+0 t = 40 |     (equation 1)
0 f+t = 20 |     (equation 2)
Divide equation 1 by 4:
{f+0 t = 10 |     (equation 1)
0 f+t = 20 |     (equation 2)
Collect results:
Answer: | f=10    and    t=20
 

Thanks, Guest.......here's a sightly easier appproach  using one variable

Call the number of 4 kilos he sold, x       and the number of 3 kilos,  30 - x     and we have

4x + 3(30 - x)  = 100   simplify

x + 90   = 100     subtract 90 from both sides

x = 10     = the number of 4 kilos sold          30 - x   = 30  - 10   = 20    =   the number of 3 kilos sold