The cost of a T-shirt was two times as much as the cost of a pair of socks. Joe bought 3 pairs of socks and 1 T-shirt for $100. Find the total cost of 1 T-shirt and 2 pairs of socks.
lets call the cost of a pair of socks $s$, and the cost of the T-shirt is $t$. we have:
we can substitute $t$ for $2s$: