Need some help with numeric distribution

In Mockaroo you can use the Normal Distribution type. You can set a mean and standard deviation to get close to what you want.

If, however, you want exactly 50% to be between 40 and 60, you’d need to do that with the Formula type. For example:

[random(40, 60), [random(0, 39), random(61, 100)].sample].sample