Answer:
£13
Explanation:
Set up a system of equations where s represents the cost of one shirt and j represents the cost of one jumper:
2s + j = 29
s + j = 21
Multiply the bottom equation by -2 to solve by elimination by cancelling out the s variables:
2s + j = 29
-2s - 2j = -42
-j = -13
j = 13
One jumper will cost £13