The probability distribution of a random variable X is given below:

x	1	2	3	4	5	6
$P(X=x)$	a	a	a	b	b	0.3

If mean of X is 4.2, then  $2a+3b$  is equal to 

$3a+2b+0.3=1$

$\Rightarrow 3a+2b=0.7$             ………….. (1)

$a+2a+3a+4b+5b+1.8=4.2$

$6a+9b=2.4$                   ………… (2)

From (1) and (2) $a=0.1;b=0.2$