I would use the method of rounding because it says to estimate.
Estimate=Round
Because 4.6 is closer to 5 than it is to 4 and 3.9 is closer to 4 than it is to 3, I would just multiply 5 times 4 which equals 20 (give or take a few, but because we are estimating, it’s ok).
Answer: ~20
P.S. the squiggly line is optional. It just symbolizes/means give or take a few when using rounding/estimate.