Let flat fee for call be X and rate per minute be Y.
So our equation for 10 minutes of call becomes X + 10Y = 3.19 ------------(1)
And X + 15y = 4.29 for 15 minutes------------------------(2)
Subtracting equations (2) from (1)
5y=1.10
Y = 1.10/5 = 0.22
Substituting value for y In (1) we have
X + 10(0.22) = 3.19
X = 3.19 - 2.2 = 0.99
Since we have obtained flat fee for call and rate per minute
The cost for 12 minute call
X + 12Y
0.99 + 12(0.22)=3.63
The fee for 12 minuter call =$ 3.63