An ideal ammeter (zero resistance) and an ideal voltmeter (infinite resistance) are connected as shown. The ammeter reading  is given asR1=5Ω,R2=15Ω,R3=1.25Ω and E=20 V

Req=R1R2R1+R2( Parallel combination) ​Req=R1+R2 Series combination ​Req of the circuit =5×155+15+125100=7520+54=5Ω We know that I=EReq=205=4 A​ Potential difference across R1 and R2 are same  (Parallel combination) I1R1=4−I1R2​⇒5I1=4−I1×15⇒I1=12−3I1⇒I1=3 A​ Thus reading of ammeter =3 A​ Voltage across 1.25Ω=I×R=4×1.25=5 V (Reading of voltmeter)