Mika would like to estimate how far she can ride a new model of electric bike on a fully charged battery. She completed two trips totaling $40$ miles. The first trip used $\frac{1}{2}$ of the total battery power, while the second trip used $\frac{3}{10}$ of the total battery power. How many miles can this electric bike go on a fully charged battery?

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Written solution:

The two trips used $\frac12+\frac3{10}=\frac45$ of a full battery. Since $\frac45$ of the range is $40$ miles, a full charge gives $40\div\frac45=50$ miles.