75% of 7500=5625 and 45% of 7500=3375.
Let E=number of English only candidates and M=number of maths only candidates. Let X be the number offered both, then E+X+M=7500; we know that E+X=5625 and M+X=3375. Therefore E+X+M+X=(E+M+X)+X=5625+3375=9000. We know E+M+X=7500, so 7500+X=9000, and X=1500.
Therefore 1500 candidates were offered both. [Also 4125 were offered English only and 1875 were offered maths only.]