Use the flex Shorthand Property

There is a shortcut available to set several flex properties at once. The flex-grow, flex-shrink, and flex-basis properties can all be set together by using the flex property.

For example, flex: 1 0 10px; will set the item to flex-grow: 1;, flex-shrink: 0;, and flex-basis: 10px;.

The default property settings are flex: 0 1 auto;.

Add the CSS property flex to both #box-1 and #box-2. Give #box-1 the values so its flex-grow is 2, its flex-shrink is 2, and its flex-basis is 150px. Give #box-2 the values so its flex-grow is 1, its flex-shrink is 1, and its flex-basis is 150px.

These values will cause #box-1 to grow to fill the extra space at twice the rate of #box-2 when the container is greater than 300px and shrink at twice the rate of #box-2 when the container is less than 300px. 300px is the combined size of the flex-basis values of the two boxes.