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Fibonacci Deduction

Stage: 3 and 4 Short Challenge Level: Challenge Level:1

Leonard writes down a sequence of numbers. After the first two numbers, each number is the sum of the previous two numbers in the sequence. The fourth number is $6$ and the sixth number is $15$. What is the seventh number in the sequence?
If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas. 

This problem is taken from the UKMT Mathematical Challenges.
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