Your mother did had significant amount os money to invest in the property, yet the property was purchased by taking a home loan . Presuming she is eligible for claiming exemption u/s 54 (Kindly refer to the provision) she should invest the entire amount of capital gain , so that she can get exemption
Cost of the new property is 62 Lacs
Your contribution is of 45 lacs
That leaves her contribution at 17 lacs
Exemption shall be restricted to 17 Lacs only and she will have to pay tax on 35 lacs - 17 lacs @ 20% (+ cess etc)
Due to time constraints I am not citing any case laws, but there are several HC judgement which denies exemption if the money has not been invested by the assessee.
It has been presumed that property which your mother owned was a residential house property
If the exemption is to be availed u/s 54F and not S 54 , then note that entire sale consideration should have been invested to cover entire capital gain.
Sale consideration 60 Lac (presuming it to be net sale considertion, after cost of transfer, brokerage etc)
ICOA 25 Lacs
Cap gain 35 Lacs
Exemption 17/60 * 35 lacs =9.9167 Lacs
Taxable = 35 Lacs - 9.9167 lacs = 25.083 Lacs
Exemption can be claimed u/s 54EC [Bonds of NHAI/RECL] SIMULTANEOUSLY