Solve these simultaneous equations:

(a) a + b = 3 and a − b = 5

(b) 2x + 3y = 27 and 3x + 2y = 23

(c) 2x + 2y + 4z = 24 and 6x + 3y = 15 and y + 2z = 11

(d) x + y − 2z = −2 and 2x − y + z = 9 and x + 3y + 2z = 4

(e) 4r − 2s + 3t = 12 and r + 2s + t = −1 and 3r − s − t = −5