Key idea: Arithmetic Progression (AP) in motion problems.
This is recognisable because distances covered in successive equal time intervals (each minute) increase by a constant amount, requiring the sum of an AP formula to find total distance.
Step 1: Formulate Usain's distance.
Usain's minute-by-minute distances form an AP: 70,100,130,…
First term aU=70, common difference dU=30.
Distance covered by Usain in n minutes: SU(n)=2n[2(70)+(n−1)30]=15n2+55n.
Step 2: Formulate Ani's distance.
Ani's distances form an AP: 100,150,200,…
First term aA=100, common difference dA=50.
Distance covered by Ani in k minutes: SA(k)=2k[2(100)+(k−1)50]=25k2+75k.
Step 3: Account for the time offset.
Ani starts 3 minutes later. When Ani has run for k minutes, Usain has run for k+3 minutes.
They meet when their total distances are equal: SU(k+3)=SA(k).
Step 4: Solve for k.
15(k+3)2+55(k+3)=25k2+75k
15(k2+6k+9)+55k+165=25k2+75k
15k2+90k+135+55k+165=25k2+75k
15k2+145k+300=25k2+75k
10k2−70k−300=0
Divide by 10: k2−7k−30=0.
Factor: (k−10)(k+3)=0.
Since k>0, k=10.
Answer: 10