IIRC, you can mix speeds and the memory will run at the lowest speed. Keep in mind that your motherboard needs to support both speeds.
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All that is gold does not glitter,
Not all those who wander are lost;
The old that is strong does not wither,
Deep roots are not reached by the frost.
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If you give a man a fish, he will eat a day;
But if you teach a man to fish, he will buy an ugly hat;
And if you talk about a fish to a starving man, then you're a consultant