Total number of students = $$1800$$
Students opted basketball = $$750$$
Students opted cricket = $$800$$
Students opted table tennis -
= $$1800$$ - ($$750$$ + $$800$$)
= $$1800$$ - $$1550$$
= $$250$$
Students opted tennis = $$250$$
Finding Ratios -
Students who opted basketball : Students opted table tennis
$$750$$ : $$250$$
Simplifying by $$250$$
$$250$$ x $$3$$ = $$750$$
$$250$$ x $$1$$ = $$250$$
$$3$$ : $$1$$
The Ratio is $$3$$ : $$1$$