Eyeballing that log graph makes it looks like the 1Msun point corresponds to a relative number of about 15 (*). I don't know if that's where you're getting stuck because you didn't explain exactly what part of the question you are having trouble with.
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Looks like around
4.50e4 / 20 ≈ 2250 10-M suns. CORRECTED ESTIMATE
Eyeballing that log graph makes it looks like the 1Msun point corresponds to a relative number of about 15 (*). I don't know if that's where you're getting stuck because you didn't explain exactly what part of the question you are having trouble with.
For every 1000 stars, about 125 will be 10 x solar mass.