Question 259964
(s+t)/2  = 12  ----------1
(s+t+u)/3  = 15 -------- 2

s+t= 12*2=24 from equation 1
substitute the vale of s+t in equation 2

(24+u) / 3  = 15
24+u = 15*3=45
u=45-24
u=21