An ideal ammeter (zero resistance) and an ideal voltmeter (infinite resistance) are connected as shown. The ammeter reading  is given as${\mathrm{R}}_1=5\Omega ,{\mathrm{R}}_2=15\Omega ,{\mathrm{R}}_3=1.25\Omega$ and E=20 V

$\begin{array}{l}{\text{R}}_{eq}=\frac{{\text{R}}_1{\text{R}}_2}{{\text{R}}_1+{\text{R}}_2}(\text{ Parallel combination) }\\ {\text{R}}_{eq}={\text{R}}_1+{\text{R}}_2\left(\text{ Series combination }\right)\\ {\text{R}}_{eq}\text{ of the circuit }=\frac{5\times 15}{5+15}+\frac{125}{100}=\frac{75}{20}+\frac{5}{4}=5\Omega \end{array}$

$\begin{array}{l}\text{ We know that I}=\frac{\text{E}}{{\text{R}}_{eq}}=\frac{20}{5}=4 \text{A}\\ {\text{ Potential difference across R}}_1{\text{ and R}}_2\text{ are same (Parallel combination) }\\ {\text{I}}_1{\text{R}}_1=\left(4-{\text{I}}_1\right){\text{R}}_2\text{}\\ \Rightarrow 5{\text{I}}_1=\left(4-{\text{I}}_1\right)\times 15\\ \Rightarrow {\text{I}}_1=12-3{\text{I}}_1\\ \Rightarrow {\text{I}}_1=3 \text{A }\\ \text{Thus reading of ammeter }=3 \text{A }\\ \text{Voltage across }1.25\text{Ω}=\text{I}\times \text{R}=4\times 1.25=5 \text{V (Reading of voltmeter)}\end{array}$