A special tea is a mixture of three kinds of tea leaves A, B and C in the ratio 5 : 3 : 2.1 kg of each kind of tea leaves costs $4, $20 and $10 respectively. What is the cost of 15 kg of this special tea?

Answer 1

Trying to make sense of your typo
I think you meant your ratio was 5:3:2
and then: 1 kg of each kind .....
(looks like the ratio is 5:3:2.1, but then the next part of the sentence
makes no sense)

I read it this way:

ratio of tea mixture:
A:B:C = 5:3:2 = 5x : 3x : 2x
let's assume that we have 1 kg, that is,
9x = 1
x = 1/9
so we have 5/9 kg of A, 3/9 kg of B, and 2/9 kg of C
cost of 1 kg = (5/9)(4) + (3/9)(20) + 2/9(10) dollars
= $(20+60+20)/9
= $100/9

So 15 kg would cost 15(100/9) or $166.67

Answer 2

5+3+2 = 10 parts of tea
so to make 15 kg of the mix, you will need 3/2 as much of each part. That is,
7.5 + 4.5 + 3 kg of each
Then the cost will be
7.5*4 + 4.5*20 + 3*10 = $150