Answer
The cost of one shirt is $10.
The cost of one pair of pants is $20.
Explanation
Let the price of one shirt be s dollars.
Let the price of one pair of pants be p dollars.
Barney spent $70 on 3 shirts and 2 pairs of pants
3s + 2p = 70
Robin bought 1 shirt and 4 pairs of pants for $90
s + 4p = 90
We can then write the two equations together and solve them simultaneously.
3s + 2p = 70
s + 4p = 90
Solving these simultaneous equation, we have
s = 10 dollars
p = 20 dollars
Hope this Helps!!!