15% is more than 10% which is $2.42 Just move the decimal to the left by 1 decimal place

So you know 15% is more than $2.42

15% is less than 20% which is twice 10% or $4.84

15% is in the middle between $2.42 and $4.84

That's a little less than between 2 1/2 and 5 dollars. That's a little less than half of 7 1/2 dollars or $3.75

Estimate it as $3.50 which is a little less than $3.75

Actual answer is 10% or $2.42 plus half of 10% or $2.42+$1.21 = $3.53

An estimate of $3.50 is very close to the actual $3.53

Estimates tend to be whole integers, rounded off to a dollar multiple or quarter of a dollar

Alternatively, when you calculated 10% and 20% as $2.42 and $4.84

that's close to $2.40 and $4.80 In the middle is about 15% or $3.60. But you know it's a little less than that, as you rounded off downward on the initial 10 and 20%, so maybe lower it by 10 cents to get a nicer rounded $3.50

Estimates are usually found by simplifying and rounding and ignoring the precise clumsier numbers