The best thing to do is to find the cost of one sweet, and to do this, divide 42 by 7.
42/7= 6
Therefore, one sweet costs 6p.
To find the cost of 8 sweets, you've got to multiply 6 by 8, and this gives you 54p.
Therefore, 8 sweets cost 54p
This is basically just finding unit rate and multiplying.
Hope this helps :)