Ratio maths question help please.

The question is: 'a catering box of crisps has two flavours, plain and beef in the ratio 3:4. There are 42 packets of plain crisps. How many packets of beef crisps are there in the box?' How do you work this out and what is the answer?

3/4 = 42/x
cross multiply:
3x = 168
x = 168/3 = 56 packets of beef flavored crisps

- .--

3x + 4x = 42 packets

x = 6

so, there are:

18 packets of plain crisps
24 packets of beef crisps

check...I did

@ƒ

3/4 = 42/x
3x = 168
x = 56