The ammeter shown in Figure P28.20 reads 2.50 A. Find I1, I2, and the emf..?

Ignore the bottom part (2Ω and "E") and calculate the current I1 through the 7Ω and 5Ω resistors from the 15V source. Subtract the value from the 2.5A (I1+I2) the meter is showing and using the new value I2 (I2=2.5A-I1) forget about the top half of the circuit (15V and 7Ω) and calculate the voltage E using I2 and 2Ω+5Ω.