Let S be the cost of 1 shirt, C be the cost of 1 cap. We must try to eliminate either S or C from the equations, so that we can find the remaining (either S or C) easily. Here, we know that 2S=5C, since 2 shirts cost as much as 5 caps. By simply multiplying this eqn by 2, we get 4S=10C, and 10C is also present in the next equation, 7S+10C=275. Replace the 10C with 4S to get an equation with only S.