Let's let the cost of a shirt be s and the cost of a jumper be j. Then if 2 shirts and 1 jumper cost 25.35, we have 2s + j = 25.35. If 1 shirt and 1 jumper cost 18.05, then we have s + j = 18.05. Now we have two equations and also two variables (s and j). There are multiple ways to solve this, but the easiest might be the substitution method. Solve the second equation for s. Once you have that, plug that value of s into the first equation where s appears. Then you will have a single equation with only one variable (j). You can then solve that equation for j. Please let me know if you have any difficulty with the algebra.