Solve the system by substitution 0.3a + 0.1b = 1.3 4/3a + 1/3b = 6

0.3a+0.1b=1.3
4/3a+1/3b=6

First make the second problem into decmoals.
4.3a+.3b=6

Now move the 0.3a to the other side of the equal sign by subtracting
0.1b=1.3-0.3a

Now divide the 0.1 on both sides
b=13-3a

Now substitute the b in the second problem for 13-3a 
4.3a+.3(13-3a)=6

Multiply the .3 with 13 and -3
4.3a+3.9-.9a=6

Subtract the 3.9 on both sides
4.3a-.9a=2.1

Subtract the 4.3a with the .9a
3.4a=2.1

Now divide 3.4 on both sides 
a=.61

Now substitute the a in the first problem for .61
b=13-3(.61)

Multiply the -3 with .61
b=13-1.83

Now subtract 13 and 1.83
b=11.17

Now your answer is
a=.61
b=11.17