From the given data,

$$\begin{gathered} 40 100 x 75 + 80 100 x 25 = K 100 x 250 \\ 30 + 20 = 5 K 2 \\ 50 x 2 = 5 K \\ K = 100 5 = 20 \end{gathered}$$

20% of 120

20 x 120/100

= 4 x 6

= 24

Hence, 20% of 120 = 24.

6% of 350000

$$\begin{gathered} 6 100 x 3 , 50 , 000 \\ = 6 x 3500 \\ = 21 , 000 . \end{gathered}$$

Hence, 6% of 3,50,000 = 21,000.

Mass of zinc ore = 8000 kg

Mass of metal = 60% of 8000 = 4800 kg

Mass of silver in metal = 3/4 x 4800/100 = 36 kg

Mass of zinc = 4800 - 36 = 4764 kg.

The given sentence 21 is 35 of what number means that 

21 is 35 % of what number

Let that number be P

=> 35P/100 = 21

35P = 2100

P = 2100/35 = 300/5 = 60

Hence, 21 is 35 of 60.