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Hi,
When Lily was 3 months old, she weighed 10 pounds.
She is gaining about 3/4 of a pound each month.
How much should she weigh after 18 months?
Let x represent the number of months:
weight = (3/4)(x-3) + 10lb
weight at 18mo = (3/4)(18-3) + 10lb = 21.25lb 0r 21 1⁄4 lb