The sum of a triangle's angles is always 180°. Since the other two measure 42° and 30°, the sum is 72° so you just substract 72° from 180° and get 108°.
Amerchant has coffee worth $60 a pound that she wishes to mix with 50 pounds of coffee worth $90 a pound to get a mixture that she will sell for $70 a pound. how many pounds of the & 60 coffee should be used?