Derek finally gets his own allowance. He puts $2 away in January, $4 away in February, $8 away in March, $16 away in April and follows this savings pattern through to December. How much money does he have to spare and save by December?
Let's be accurate as possible and think first.
4096
Derek doubles his allowance savings each month, so in May, Derek has $16 * 2 = $32.

In June, Derek has $32 * 2 = $64.

In July, Derek has $64 * 2 = $128.

In August, Derek has $128 * 2 = $256.

In September, Derek has $256 * 2 = $512.

In October, Derek has $512 * 2 = $1024.

In November, Derek has $1024 * 2 = $2048.

By December, Derek has $2048 * 2 = $4096 to save.

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The answer: 4096